This disclosure relates to dental burs, in particular to dental burs formed by a plurality of solid layers, such as formed by certain additive manufacturing processes. The disclosure also relates to methods of making such dental burs, and to data for control of additive manufacturing devices.

Certain additive manufacturing methods, such as selective laser sintering or selective laser melting, form articles from solid layers that are obtained by repeating a process of melting powder particles of a specific composition via a laser beam or electron beam, thereby fusing the particles to other particles in the same layer and to the layer underneath obtained in a previous step, applying another layer of powder particles over the previous, now solidified layer, melting certain of those powder particles via the beam and fusing them to other particles in the same layer and to the layer underneath, and so on.

Abrasive articles can be obtained via these methods by using a powder particle mixture comprising metallic binder particles and abrasive particles. A suitable process is described in the International Patent Application published as WO 2018/160297 Al. The resulting metal bond abrasive article has abrasive particles retained in a matrix of metallic binder.

In rotary abrasive dental tools such as dental drill bits, dental burs or dental polishing tools, the tool head may be an abrasive article made by these additive manufacturing methods. A certain number of abrasive particles per unit volume is required at the abrading surface portion of the tool head to make the tool head sufficiently abrasive for efficient removal of material. The abrasive tool and the tool head are generally supposed to be small, so that they can be used in a patient’s mouth. In spite of its small size, the mechanical stability of the tool head is important: Where the dentist applies higher mechanical forces on the tool to drill, grind or polish faster, the tool head must not break easily.

In an attempt to address these needs, the present disclosure provides, in a first aspect, a metal bond abrasive dental bur, comprising a tool head, wherein the tool head is formed by a plurality of solid layers of a metallic binder material having abrasive particles retained therein, the solid layers being directly bonded to each other, and wherein the tool head comprises an abrading surface portion, rotationally symmetric with respect to a symmetry axis defining axial directions and radial directions, a radially outer portion comprising the abrading surface portion, and a radially inner portion through which the symmetry axis passes, characterized by the average number of abrasive particles per unit volume of the tool head being higher in the radially outer portion than in the radially inner portion.

In metal bond abrasive articles, the metallic binder provides mechanical stability to the tool head, while the abrasive particles, embedded in the binder, do not add to the mechanical stability, but rather form weak portions in the tool head. The abrasive particles are mostly selected to be hard and rigid (i.e. non-elastic) to exhibit good abrasive properties. The tool head, on the other hand, is mechanically more stable if it is elastic. While it is thus generally desirable to have a lower number of abrasive particles per unit volume of the tool head to increase the mechanical stability of the tool head, it is also desirable to have a higher number of abrasive particles per unit volume of the tool head to increase the abrasivity of the tool head, i.e. its capability to abrade material.

Where abrasive particles are homogenously distributed in a tool head, the number of abrasive particles per unit volume of the tool head must be carefully selected to find an appropriate balance between sufficient mechanical stability and sufficient abrasivity of the tool head.

Traditional additive manufacturing of metal bond abrasive articles could only make tool heads in which the abrasive particles were homogenously distributed.

In metal bond abrasive articles according to the present disclosure, however, the abrasive particles are not homogenously distributed in the tool head, but their concentration (i.e. their number per unit volume of the tool head) is higher in a radially outer portion of the tool head than in a radially inner portion. As a result, the inner portion has a higher mechanical stability, and the outer portion provides appropriate abrasivity, so that higher force can be used for drilling, grinding or polishing and the target material can be abraded faster. The metal bond abrasive article of the present disclosure is an abrasive article in which abrasive particles are distributed and supported in a metallic matrix, namely the metallic binder material. Specifically, the metal bond abrasive article comprises a tool head in which abrasive particles are retained in a metallic binder material.

Metal bond abrasive articles according to the present disclosure are metal bond abrasive dental burs. Other metal bond abrasive items may have the same structure as the metal bond abrasive dental burs described herein, such other metal bond abrasive items may be, for example, abrasive pads, abrasive grinding bits, abrasive segments, abrasive wheels, or other rotary abrasive dental tools (e.g., dental drill bits, or dental polishing tools).

In a metal bond abrasive article according to the present disclosure, the tool head is formed by a plurality of solid layers of metallic binder material having abrasive particles retained therein, the solid layers being directly bonded to each other. The tool head may thus be made, for example, using processes known as layer-by-layer manufacturing or using processes known as additive manufacturing processes.

In layer-by-layer manufacturing methods, one layer of metallic binder material having abrasive particles retained therein is built on a previous layer, and this process is repeated to form the tool head. The deposition of one layer on the previous layer may be done, for example, in a bath of chemicals. Each layer may be dried and cured before the next layer is deposited in the bath.

In additive manufacturing processes, like in one generally known as “selective laser melting” or SLM, an article is manufactured by depositing a layer of loose powder particles in a build region and irradiating selected lines or points of the layer with a focused laser beam or particle beam to bond powder particles together, repeating many times the steps of depositing the powder layer and irradiating lines of it to melt and bond powder particles. The molten and bonded irradiated areas of the respective layers form the article, while the loose, unirradiated powder is removed. A so-called dressing process is then performed to expose buried abrasive particles on the abrading surface portion. This may be described as a “sharpening” process in which a dressing tool, which is softer than the abrading surface portion, removes some of the metallic binder material, while the abrasive particles stay in place and become exposed on the abrading surface portion. The result of such an additive manufacturing process is an article, e.g. a metal bond abrasive article or a tool head according to the present disclosure, that is formed by a plurality of solid layers which are directly bonded to each other.

The solid layers forming the tool head of a metal bond abrasive article according to the present disclosure may have a thickness of between 10 pm and 1000 pm. In preferred embodiments the solid layers have a thickness of between 20 and 100 pm. In certain of these embodiments the solid layers have a thickness of 25 pm or of 50 pm.

According to the present disclosure, the tool head has an abrading surface portion. This surface portion contacts the workpiece that is supposed to be polished, drilled or ground. In this surface portion, portions of abrasive particles protrude from the matrix of metallic binder material. When the abrading surface portion moves (e.g. rotates) relative to the workpiece, the protruding portions remove material from the workpiece. Over time, also the tool head will be abraded, so that protruding abrasive particles may be removed from the abrading surface portion, and portions of other abrasive particles, previously completely buried in the matrix of metallic binder material, will protrude from the abrading surface portion and provide abrasivity to the tool head.

The abrading surface portion is rotationally symmetric with respect to a symmetry axis. This makes the abrading surface portion and the tool head usable in rotary abrasive applications, e.g. in rotary abrasive dental applications. The symmetry axis defines axial directions, which are directions along the symmetry axis or parallel thereto, and radial directions, which are directions orthogonal to the symmetry axis.

A tool head of a metal bond abrasive article according to the present disclosure comprises a radially outer portion which comprises the abrading surface portion. A “radially outer” portion is a portion of the tool head that is located further from the symmetry axis, measured in a radial direction.

A tool head of a metal bond abrasive article according to the present disclosure comprises a radially inner portion through which the symmetry axis passes. A “radially inner” portion is a portion of the tool head that is located closer to the symmetry axis, measured in a radial direction. In particular, a radially inner portion is located closer to the symmetry axis, measured in a radial direction, than a radially outer portion. According to the present disclosure, the number of abrasive particles per unit volume of the tool head is higher in the radially outer portion than in the radially inner portion. The number of abrasive particles per unit volume is also referred to herein as the “concentration” of abrasive particles. The concentration may be determined, for example, by tomography using x-rays or gamma rays.

In order to compare the particle concentration of the inner portion with the concentration of the outer portion, it is generally sufficient to count abrasive particles in a cross section of the tool head. Such a cross section may be obtained by cutting through the tool head in a radial direction and polish the new sectional area. Subsequent imaging of the sectional area using scanning electron microscopy or a similar suitable technique, including suitable illumination, may allow to count the abrasive particles in the cross section per unit area (e.g. per 0.2mm x 0.2mm or per 0.5mm x 0.5mm) of the cross section. A higher count per unit area of the cross section generally corresponds to a higher number of particles per unit volume of the tool head, and a lower count per unit area of the cross section generally corresponds to a lower number of particles per unit volume of the tool head. For determining if a particle concentration in one portion is higher or lower than a particle concentration in another portion, such a cross section counting method is an adequate method.

The tool head comprises a metallic binder material. The metallic binder material may comprise cobalt, chromium, bronze, copper, tin, iron, an iron alloy, silver, nickel, tungsten, titanium, manganese, aluminium, silicon, their carbide or nitride forms, or combinations thereof. In certain embodiments, the metallic binder material further comprises an aluminium alloy, copper, a copper-silver alloy, a copper-phosphorus alloy, a nickel-phosphorus alloy, or a brazing alloy containing silver.

The abrasive particles may comprise any abrasive particles used in the abrasives industry. Preferably, the abrasive particles have a Mohs hardness of at least 4, preferably at least 5, more preferably at least 6, more preferably at least 7, more preferably at least 8, more preferably at least 8.5, and more preferably at least 9.

In certain embodiments, the abrasive particles comprise superabrasive particles. As used herein, the term “superabrasive” refers to any abrasive particle having a hardness greater than or equal to that of silicon carbide (e.g., silicon carbide, boron carbide, cubic boron nitride, and diamond). Specific examples of suitable abrasive materials include aluminium oxide (e.g., alpha alumina) materials (e.g., fused, heat-treated, ceramic, and/or sintered aluminium oxide materials), silicon carbide, titanium diboride, titanium nitride, boron carbide, tungsten carbide, titanium carbide, aluminium nitride, diamond, cubic boron nitride, garnet, fused alumina-zirconia, sol-gel derived abrasive particles, cerium oxide, zirconium oxide, titanium oxide, and combinations thereof.

Examples of sol-gel derived abrasive particles can be found in U.S. Patent No. 4,314,827 (Leitheiser et al.); U.S. Patent No. 4,623,364 (Cottringer et al.); U.S. Patent No. 4,744,802 (Schwabel); U.S. Patent No. 4,770,671 (Monroe et al.); and U.S. Patent No. 4,881,951 (Monroe et al.). Agglomerate abrasive particles that comprise finer abrasive particles in a vitreous bond matrix (e.g., as described in U.S. Patent No. 6,551,366 (D'Souza et al.)) may also be used.

In select embodiments, the abrasive particles comprise diamond particles, cubic boron nitride particles, or both. In some embodiments, the abrasive particles comprise silicon carbide, boron carbide, silicon nitride, metal oxide ceramic particles, metal nitride ceramic particles, or metal carbide ceramic particles. In certain embodiments, the abrasive particles comprise diamonds and the at least one coating comprises a metal carbide. In certain embodiments, the abrasive particles comprise cubic boron nitride and the at least one coating comprises a metal nitride.

The abrasive particles optionally comprise first abrasive particles and second abrasive particles, wherein the first abrasive particles and second abrasive particles are disposed in interspersed predetermined different regions within the metal bond abrasive article. This can be advantageous when certain areas of a metal bond abrasive article require different levels of abrasion for a particular abrasive application. The different regions can be solid layers, for instance discrete solid layers applied individually using additive manufacturing. In some embodiments a solid layer comprises abrasive particles that have a different composition than the abrasive particles that are in the adjacent layer.

In certain embodiments, the abrading surface portion of a metal bond abrasive article according to the present disclosure is formed by a plurality of solid layers of a metallic binder material having abrasive particles retained therein, the solid layers being directly bonded to each other. In some of those embodiments the abrading surface portion comprises a plurality of the abrasive particles exposed suitably on the abrading surface portion to perform abrasion. Such metal bond abrasive articles may be particularly cost- effective to manufacture, for example by an additive manufacturing process or a 3-D printing process.

In certain embodiments, the tool head has a cylindrical shape defining an outer radius R. In some of those embodiments the radially outer portion is the portion of the tool head between 0.9 x R and 1.0 x R. In some of these embodiments the radially inner portion is the portion of the tool head between the symmetry axis and 0.2 x R. In some of those embodiments the radially outer portion is the portion of the tool head between 0.9 x R and 1.0 x R and the radially inner portion is the portion of the tool head between the symmetry axis and 0.2 x R or between the symmetry axis and 0.3 x R, or between the symmetry axis and 0.5 x R, or between the symmetry axis and 0.8 x R, or between the symmetry axis and 0.9 x R. Such metal bond abrasive articles may be manufactured at a lower cost due to the lower number of abrasive particles required, caused by the reduced size of the radially outer portion. Such metal bond abrasive articles may be mechanically structurally stronger because the number of abrasive particles, which may weaken the structural robustness of the tool head, per unit volume is lower in an inner portion of a larger volume.

In certain embodiments of a metal bond abrasive article according to the present disclosure, in cross section orthogonal to the symmetry axis, the radially inner portion has the shape of a circular disk, and the radially outer portion has an annular shape, arranged concentrically with the radially inner portion. Such a concentric arrangement is particularly easy to design and manufacture, and provides for an even weight distribution around the symmetry axis, about which the tool head rotates at high speed when in use.

In certain embodiments of a metal bond abrasive article according to the present disclosure, the number of abrasive particles per unit volume of the tool head in the radially outer portion is at least 2 times, 3 times, 5 times, or 10 times as high as in the radially inner portion. The higher density in the outer portion provides for an adequate abrasivity of the radially outer zone, both initially and after a certain wear. The lower particle density in the inner portion helps increase the structural robustness of the inner portion and therewith of the tool head and may reduce the consumption of abrasive particles in manufacturing the metal bond abrasive article. In certain embodiments of a metal bond abrasive article according to the present disclosure, the number of abrasive particles per unit area of a cross section of the tool head orthogonal to the symmetry axis in the radially outer portion is at least 1.5 times, at least 2 times, at least 3 times, at least 5 times, or at least 10 times as high as in the radially inner portion. The number of abrasive particles per unit area of a cross section of the tool head orthogonal to the symmetry axis is - under most circumstances - a measure of the number of abrasive particles per unit volume of the tool head. Hence again, the higher density in the outer portion provides for an adequate abrasivity of the radially outer zone, both initially and after a certain wear. The lower particle density in the inner portion helps increase the structural robustness of the inner portion and therewith of the tool head and may reduce the consumption of abrasive particles in manufacturing the metal bond abrasive article.

In certain embodiments of a metal bond abrasive article according to the present disclosure, the metallic binder material comprises one of cobalt, chromium, bronze, copper, tin, iron, an iron alloy, silver, nickel, tungsten, titanium, manganese, aluminum, silicon, their carbide or nitride forms, or combinations thereof. These materials can be used with many additive manufacturing methods or 3-D printing methods and can result in a particularly robust tool head.

In certain embodiments of a metal bond abrasive article according to the present disclosure, the abrasive particles comprise, or are, diamond particles and/or cubic boron nitride particles. Such particles have an excellent abrasivity and can provide for a long lifetime of the article in use.

In certain embodiments of a metal bond abrasive article according to the present disclosure, the abrasive particles comprise at least one coating disposed thereon, the coating comprising a metal, a metal oxide, a metal carbide, a metal nitride, a metalloid, or combinations thereof. In certain of these embodiments the at least one coating has an average thickness of 0.5 micrometers or greater. Coatings of such materials, and in particular coatings of this thickness, can reduce the amount of heat transferred to the abrasive particles during exposure to the focused beam. This may allow for using a higher- power beam to bond powder particles together reliably while leaving abrasive particles intact and not vaporized. The present disclosure also provides, in a second aspect, a method of making a metal bond abrasive dental bur as described above, the method comprising the steps, in this sequence, of a) carrying out a subprocess comprising the steps, in this sequence, of: i) depositing a layer of loose powder particles in a build region, wherein the loose powder particles comprise metallic binder particles and abrasive particles; ii) treating a first selected area of the layer of loose powder particles with a focused beam to deposit a first amount of energy per unit area to bond powder particles together to form a solid layer, and treating, before or after or while treating the first selected area, a second selected area of the layer of loose powder particles with a focused beam to deposit a second amount of energy per unit area to bond powder particles together; b) independently carrying out step a) a plurality of times to form the metal bond abrasive article from solid layers of the bonded powder particles, characterized in that the first amount of energy is low enough to not vaporize abrasive particles in the first selected area, and wherein the second amount of energy is high enough to vaporize at least a minimum percentage of the abrasive particles in the second selected area.

Metal bond abrasive articles according to the present disclosure are metal bond abrasive dental burs. Other metal bond abrasive items may be manufactured using the same method as the method of making metal bond abrasive dental burs described above, such other metal bond abrasive items may be, for example, abrasive pads, abrasive grinding bits, abrasive segments, abrasive wheels, or other rotary abrasive dental tools (e.g., dental drill bits, or dental polishing tools).

According to this aspect of the present disclosure, in a layer generation method for forming a metal bond abrasive article, abrasive particles in the first selected area are not vaporized, and at least some abrasive particles in the second selected area are vaporized. The method allows to form a metal bond abrasive article in a repetitive layering process such that in the finished article abrasive particles are present in the first selected area, and only few or no abrasive particles are present in the second selected area, because many of them were vaporized in the beam treatment step.

Where the presence of abrasive particles weakens the mechanical structure of the article, the method described above yields an article that provides greater mechanical strength in the second selected area due to the lower concentration of abrasive particles in this second area, whereas the article has an unchanged concentration of abrasive particles in the first selected area.

The first and second selected areas are areas of a layer of loose powder particles. In many additive manufacturing applications, it may be useful to arrange the first selected area of a current layer right onto the first selected area of the - now solidified - previous layer and potentially over the first selected areas of solid layers generated before (and hence underneath) the previous solid layer. Similarly, it may be useful to arrange the second selected area of a current layer right onto the second selected area of the previous - now solid - layer and potentially over the second selected areas of solid layers generated before (and hence underneath) the previous solid layer. This can result in three- dimensional portions of the article to contain (in the case of first selected areas) or to be free of (in the case of second selected areas) abrasive particles.

A metal bond abrasive article according to the first aspect of this disclosure may be manufactured using the manufacturing method according to the second aspect of the present disclosure. For that purpose, the first selected area of each layer of loose powder particles is chosen such as to form the radially outer portion of the tool head, and the second selected area of each layer of loose powder particles is chosen such as to form the radially inner portion of the tool head. This results in a tool head in which the average number of abrasive particles per unit volume of the tool head is higher in the radially outer portion than in the radially inner portion. The tool head is likely to have a greater mechanical structural strength or robustness and/or a higher abrasivity than a tool head in which the average number of abrasive particles per unit volume of the tool head is everywhere as high as in the radially outer portion.

The loose powder particles comprise metallic binder particles and abrasive particles, as described above. In order to achieve fine resolution of the article, the loose powder particles are preferably sized (e.g., by screening) to have a maximum size of 400 micrometer (pm) or less, preferably 250 pm or less, more preferably 200 pm or less, more preferably 150 pm or less, 100 pm or less, or even 80 pm or less, although larger sizes may also be used. The metallic binder particles, abrasive particles, and any optional additional particulate components may have the same or different maximum particle sizes, and the same or different particle size distribution parameters, for example D90, D76, D50, and/or Dio particle size distribution parameters. The loose powder particles may optionally further comprise other components such as, for example, pore inducers, fillers, and/or fluxing agent particles. Examples of pore inducers include glass bubbles and organic particles. In some embodiments, lower melting metal particles, when present, may also serve as a fluxing agent; for example as described in U.S. Patent No. 6,858,050 (Palmgren).

The loose powder particles may optionally be modified to improve their flowability and the uniformity of the layer spread in the build region. Methods of improving the powders include agglomeration, spray drying, gas or water atomization, flame forming, granulation, milling, and sieving. Additionally, flow agents such as, for example, fumed silica, nanosilica, stearates, and starch may optionally be added.

Before bonding, the layer of loose powder particles has a uniform thickness, or at least a substantially uniform thickness. For example, the thickness of the layer may be 50 pm or less, 40 pm or less, 30 pm or less, 20 pm or less, or 10 microns pm or less. The layer of loose powder particles may have any thickness up to about 1 millimeter (mm), as long as the focused beam can bond all the loose powder particles where it treats the layer. Preferably, the thickness of the layer of loose powder particles is from about 10 pm to about 500 pm, more preferably from about 10 pm to about 250 pm, more preferably from about 20 pm to about 250 pm.

The loose powder particles may be a mixture of particles containing cobalt- chromium particles as metallic binder particles, diamond particles as abrasive particles, and nanosilica particles as segregation-preventing particles to prevent mechanical de mixing of the metallic binder particles from the diamond particles. The following percentages provide an exemplary powder mixture which can be used for the present invention:

85.9 vol% cobalt-chromium particles, 10-45 pm size;

12.0 vol% diamond particles, 50-100 pm size;

2.1 vol% nanosilica particles, 5-50 nm size.

The build region is a region above a moving platform having a flat horizontal surface on which the abrasive article is built layer by layer. After one layer is built by bonding the loose powder particles to form a solid layer, the platform moves downward by a distance that corresponds to the thickness of the solid layer built last. This keeps the build region at the same vertical position. All portions of the build region can be covered by a layer of loose powder particles and selected areas of the build region can then be treated by the focused beam to bond particles together to form the next solid layer.

The first selected area may be, or comprise, a pattern of points and/or lines. The first selected area may be, or comprise, a pattern of lines. The lines may be of any geometric shape. The lines may be connected with each other, or they may be isolated from each other, i.e. they may be not connected with each other.

A focused beam is directed onto the first selected area of the powder layer to treat it without vaporizing the abrasive particles in the powder layer. Both lasers and electron beam (e-beam) sources are capable of emitting a focused beam of suitable irradiation energy. Suitable energy sources include for instance and without limitation fiber lasers, CO2 lasers, disk lasers, and solid state lasers. A suitable electron beam source is available under the trade designations Arcam QlOplus, Arcam Q20plus, and Arcam A2 (Arcam AB, Molndal, Sweden).

A focused beam can deposit a certain amount of energy in a unit area by having a suitable irradiation intensity and by irradiating the unit area over a suitable irradiation time and at a suitable repetition rate. The repetition rate is the number of consecutive times that the focused beam irradiates the unit area. At a repetition rate of 3, for example, the focused beam irradiates a certain spot for 3x the irradiation time. Typical useful repetition rates may be between 1 and 10. The amount of energy deposited per unit area is considered to be proportional to the irradiation intensity, multiplied by the irradiation time, multiplied by the repetition rate. Other factors, such as the degree of focus of the laser beam, or the temperature and absorptivity of the powder, that might affect the deposition of beam energy in the powder of loose particles are considered to be constant and suitably chosen by a skilled person for the intended application.

In certain embodiments, the focused beam to deposit a first amount of energy per unit area comprises a first laser irradiation intensity of a wavelength of 1064 nm (nanometer) providing an energy density of 1.2 Joules per square millimeter (J/mm ² ) or less, 1.0 J/mm ² or less, 0.5 J/mm ² or less, or 0.1 J/mm ² or less. At an irradiation time of 50 ps (microseconds) and a repetition rate of 1, this beam deposits a first amount of energy that is adequate to bond loose powder particles together and low enough to not vaporize abrasive particles in the powder. In other embodiments, the focused beam to deposit a first amount of energy per unit area comprises a first electron beam irradiation intensity providing an energy density of 1.2 J/mm ² or less (e.g., a power of up to 3,000 Watt and a beam diameter between 150-200 micrometers). At an irradiation time of 50 ps and a repetition rate of 1 such an electron beam deposits a first amount of energy per unit area that is adequate to bond together loose powder particles in the first selected area and/or in the second selected area of the layer of loose powder particles, to form a pattern of bonded powder particles, for example, by selective metal sintering or selective laser melting of the metallic binder particles and the abrasive particles. In this pattern, the abrasive particles are embedded or retained in a matrix of metallic binder material. The amount of energy deposited is low enough to not vaporize abrasive particles in the powder.

The irradiation time (e.g. 50 ps in the embodiment described above) is the time interval during which the focused beam remains directed at the same position to melt and bond powder particles. It is sometimes also referred to as spot duration. For a beam of a certain irradiation intensity, the irradiation time can be adjusted according to the properties of the loose powder particles, such as their composition, grain size, absorptivity, temperature etc. Such an adjustment is a straight-forward task for a skilled person. For a beam of a certain irradiation intensity and irradiation time, also the repetition rate can be adjusted according to the properties of the loose powder particles, such as their composition, grain size, absorptivity, temperature etc. Also, an adjustment of the repetition rate is a straight-forward task for a skilled person.

Vaporizing of an abrasive particle refers to the removal of the entire abrasive particle as such from the loose powder particles or from the bonded powder particles, e.g. by converting the abrasive particle into a gas. The first amount of energy deposited per unit area is low enough to not vaporize abrasive particles in the first selected area. The first amount of energy per unit area may be low enough to not vaporize abrasive particles in the first selected area, but high enough to bond powder particles together in the first selected area.

The first amount of energy deposited per unit area may be low enough to not vaporize any abrasive particles in the first selected area. The first amount of energy per unit area may be low enough to not vaporize more than 5% of the total number of abrasive particles in the first selected area, to not vaporize more than 10% of the total number of the abrasive particles in the first selected area, or to not vaporize more than 20% of the total number of the abrasive particles in the first selected area. The first amount of energy per unit area may be low enough to not vaporize the majority of the total number of the abrasive particles in the first selected area.

The second amount of energy per unit area is high enough to vaporize abrasive particles in the second selected area. The second amount of energy per unit area may be high enough to vaporize abrasive particles in the second selected area, and high enough to bond powder particles together in the second selected area.

The second amount of energy per unit area may be high enough to vaporize all, or essentially all, abrasive particles in the second selected area. The second amount of energy per unit area may be high enough to vaporize more than 90% of the total number of abrasive particles in the second selected area, to vaporize more than 80% of the total number of the abrasive particles in the second selected area, or to vaporize more than 70% of the total number of the abrasive particles in the second selected area. The second amount of energy per unit area may be high enough to vaporize the majority (i.e. more than 50%) of the total number of the abrasive particles in the second selected area.

The second amount of energy per unit area is higher than the first amount of energy per unit area. To obtain a focused beam that deposits a second amount of energy per unit area, the beam depositing the first amount of energy described above may be modified by increasing its irradiation intensity (e.g. by turning up its power) and/or by increasing the irradiation time, e.g. by prolonging the irradiation time, and/or by increasing its repetition rate. For certain mixtures of powder particles, a laser beam of irradiation intensity of approximately 50 Watt output, an irradiation time of 50 ps and a repetition rate of 1 deposits a first amount of energy to bond particles together, and is low enough to not vaporize abrasive particles. By tripling the repetition rate to 3, the same laser beam deposits a second, higher amount of energy per unit area to bond particles together which is high enough to vaporize at least a minimum percentage of the abrasive particles in the second selected area.

Obtaining a focused beam to deposit the second amount of energy may also be achieved by increasing the repetition rate to 4, 5, or 6 or to even higher values. Obtaining a focused beam to deposit the second amount of energy may also be achieved by increasing the irradiation intensity and/or by increasing the irradiation time by a sufficient amount to vaporize at least a minimum percentage of the abrasive particles in the second selected area.

The percentage of vaporized abrasive particles may be determined by comparing typical cross sections of the first (or second) selected area of a metal bond abrasive article before and after treatment with the focused beam, and counting the number of abrasive particles appearing in a unit area of the cross section, and dividing the respective numbers, as a skilled person would do.

Alternatively, the percentage of vaporized abrasive particles may be determined by comparing, in a cross section of a previously homogenous metal bond abrasive article after treatment with the focused beam of a first (or second) amount of energy per unit area in a first (or second) area, the number of abrasive particles per unit area in the untreated area with the number of abrasive particles per unit area in the treated area of the cross section, and dividing the respective numbers, as a skilled person would do.

In certain embodiments of the method of making a metal bond abrasive article according to the second aspect of the present disclosure, the second amount of energy is at least two times, at least three times, at least five times, or at least ten times as high as the first amount of energy. This may allow to ensure that abrasive particles are reliably not vaporized (or that reliably only a low percentage of abrasive particles are vaporized) in the first selected area, and to ensure that abrasive particles are reliably vaporized (or that reliably a high percentage of abrasive particles are vaporized) in the second selected area. This results in a structurally more robust and/or higher abrasive tool head.

In certain embodiments of the method of making a metal bond abrasive article according to the second aspect of the present disclosure, in each step a) the first selected area is independently selected, and the second selected area is independently selected. In each step a) of depositing a layer of loose powder particles and of treating a first and a second selected area, as described above, the first selected area may be independently selected. In each step a) the first selected area may be selected independently of the second selected area selected in the same step a) or of the second selected area selected in the previous performance of step a). The first selected area may be selected independently of the first selected area selected in the, or a, previous performance of step a).

Similarly, in each step a), the second selected area may be independently selected. The second selected area may be selected independently of the first selected area selected in the same step a) or of the first selected area selected in the previous performance of step a). The second selected area may be selected independently of the second selected area selected in the previous performance of step a).

The independent selection of the first and second selected areas may help provide that the method can be used to make metal bond abrasive articles which can have many different shapes and many different internal structures. The independent selection may allow the pattern of one solid layer of the article being independent from the pattern of the previously generated solid layer of the article, and hence may allow for a greater freedom in forming geometrical shapes.

In certain embodiments of the method of making a metal bond abrasive article according to the second aspect of the present disclosure, the layer of loose powder particles has a uniform thickness. The uniform thickness may help to obtain a more reliable bonding of powder particles and/or to have better control over the boundaries of the first and second selected areas and over the controlled vaporization of abrasive particles in the second selected area.

In certain embodiments of the method of making a metal bond abrasive article according to the second aspect of the present disclosure, the focused beam is a laser beam or a particle beam, such as an electron beam. Laser beams and particle beams deliver appropriate amounts of energy and can be switched on and off at high frequencies, resulting in better control of the treatment and subsequent bonding of loose powder particles.

Like many additive manufacturing methods, the method of making a metal bond abrasive article according to the second aspect of the present disclosure can be executed using a computer. Hence the present disclosure provides, in a third aspect, a data stream comprising computerized instructions which, when executed on a processor operationally connected to an additive manufacturing device, can cause the additive manufacturing device to perform a method of making a metal bond abrasive article as described herein.

The disclosure will now be described in more detail with reference to the following Figures exemplifying particular embodiments:

Fig. 1 Perspective view of a metal bond abrasive article according to the present disclosure; Fig. 2Raster electron microscope image of a cross section of a tool head of a metal bond abrasive article according to the present disclosure; and

Fig. 3 Sketch of an additive manufacturing process suitable to perform the method of making a metal bond abrasive article according to the present disclosure.

The perspective view of Figure 1 illustrates a metal bond abrasive article 1 according to the present disclosure. The article l is a dental bur 1 which comprises a shank 20 and a cylindrical tool head 10, secured to the shank 20. The tool head 10 comprises a plurality of abrasive particles 30, retained in a metallic binder material 40. The abrasive particles 30 are retained not only on the surface of the tool head 10 but are distributed in the volume of the tool head 10, i.e. in its bulk material. In use, the dental bur 1 rotates about a symmetry axis 50 for the tool head 10 to abrade dental material from a tooth. The symmetry axis 50 defines axial directions 100, parallel to the symmetry axis 50, and radial directions 110, i.e. directions orthogonal to the axial directions 100.

The tool head 10 has an abrading surface portion 60 on its radially outer surface of its cylindrical shape. The distribution of the abrasive particles 30 in the tool head is not homogenous, as can be seen on the top face 70 of the tool head 10: the tool head 10 comprises a radially outer portion 80 and a radially inner portion 90, and the number of abrasive particles 30 per unit volume of the tool head 10, i.e. the density of abrasive particles 30 in the tool head, is higher in the radially outer portion 80 than in the radially inner portion 90.

The tool head 10 is formed by a plurality of solid layers 120, of which only the lowest two are shown in Figure 1. The tool head 10 is built layer upon layer in an additive manufacturing process, starting from the lowest layer 120, building a further layer 120 upon the lowest layer 120, and adding further layers 120 until the tool head 10 is complete. Each layer 120 contains metallic binder material 40 with abrasive particles 30 retained therein and each layer is directly bonded to the layer 120 underneath, which is the layer 120 built in the previous step. These additive manufacturing techniques are known in the art and are becoming more widely used. A more detailed description of suitable additive manufacturing processes for this type of metal bond abrasive articles can be found in the PCT patent application published as WO 2018/160297 Al. The layers 120 are indicated in Figure 1 in dashed lines only, because the finished article 1 will not exhibit easily visible signs of layers 120.

Figure 2 is an Electron Microscope image of a cross section of a tool head 10 of a metal bond abrasive article 1 like the one shown in Figure 1. The image plane is a plane orthogonal to the symmetry axis 50. The diameter of the tool head 10, indicated by the outer circle 130, is about 2.5 mm. Image elements outside this outer circle 130 are imaging artefacts and are not part of the tool head 10.

The metallic binder material 40 appears as bright areas in the image, whereas the abrasive particles 30 retained in the metallic binder material 40 appear as dark spots.

In this cross-sectional view, the radially inner portion 90 has the shape of a circular disk, centred around the symmetry axis 50. In this view, the radially outer portion 80 has an annular shape and surrounds the radially inner portion 90. The outer edge of the outer portion 80, indicated by the outer circle 130, is the abrading surface portion 60 of the tool head 10. The inner circle 140 is drawn into the Electron Microscope image to indicate the boundary between the radially inner portion 90 and the radially outer portion 80. At this boundary, the radially inner portion 90 and the radially outer portion 80 contact each other. Both portions 80, 90 contain identical metallic binder material 40, and only the number of abrasive particles per unit volume of the tool head 10 changes at the inner circle 140: this number is higher in the radially outer portion 80 than in the radially inner portion 90, as can be seen from the density of dark spots in the cross sectional view: In the radially outer portion 80 there are more dark spots per unit area (e.g. per square centimeter) of the cross section than there are in the radially inner portion 90.

Figure 3 schematically depicts an exemplary additive manufacturing process used in making a metal bond abrasive article according to the present disclosure. In the first step, a pusher 230 horizontally pushes a quantity of loose powder particles 210 from a first powder chamber 220a and vertically movable first piston 222a into the second powder chamber 220b above the top surface of a vertically movable second piston 222b to form a layer 138 of loose powder particles 210, deposited in a build region 140 in the second powder chamber 220b. The loose powder particles 210 comprise metallic binder particles 40 and abrasive particles 30. The layer 138 should be of substantially uniform thickness. The thickness of the layer 138 of loose powder particles 210 may be, for example, 50 pm or less, 40 pm or less, 30 pm or less, 20 pm or less, or 10 pm or less. The layer 138 may have any thickness up to about 1 millimeter, as long as the focused beam can bind all the loose powder particles 210 where it is applied. Preferably, the thickness of the layer 138 is from about 10 pm to about 500 pm, more preferably about 10 pm to about 250 pm, more preferably about 20 pm to about 250 pm. In order to achieve fine resolution, the loose powder particles 210 are preferably sized (e.g., by screening) to have a maximum size of 400 pm or less, preferably 250 pm or less, more preferably 200 pm or less, more preferably 150 pm or less, 100 pm or less, or even 80 pm or less, although larger sizes may also be used. The metallic binder particles 40, the abrasive particles 30, and any optional additional particulate components may have the same or different maximum particle sizes, and the same or different particle size distribution parameters, for example D90, D76, D50, and/or Dio particle size distribution parameters.

Generally, the loose powder particles 210 may optionally further comprise other components such as, for example, pore inducers, fillers, and/or fluxing agent particles. Examples of pore inducers include glass bubbles and organic particles. In some embodiments, lower melting metal particles, when present, may also serve as a fluxing agent, for example as described in U.S. Pat. No. 6,858,050 (Palmgren). The loose powder particles 210 may optionally be modified to improve their flowability and the uniformity of the layer spread. Methods of improving the powders include agglomeration, spray drying, gas or water atomization, flame forming, granulation, milling, and sieving. Additionally, flow agents such as, for example, filmed silica, nanosilica, stearates, and starch may optionally be added.

Next, a focused beam 170 is directed onto the predetermined selected area 180 of layer 138. Typically, the focused beam 170 is provided by coupling an energy source 160 with a mirror system 150, e. g. a galvo mirror scanner. Both lasers and electron beam (e- beam) sources are capable of emitting a suitable focused beam of energy. Suitable energy sources 160 include for instance fiber lasers, CO2 lasers, disk lasers, and solid-state lasers.

Referring again to Figure 3, the focused beam 170 bonds together the loose powder particles 210 in at least one selected area 180, 181 of the loose powder particles 210 to form a layer 120 of bonded powder particles in the selected area 180, for example, by selective metal sintering or selective laser melting of the metallic binder particles 40 with the abrasive particles 30 retained therein. The above steps are then repeated to form a next layer 120, potentially with changes to the selected area 180, 181 where the beam 170 is focused according to a predetermined design resulting through repetition, layer 120 on layer 120, in a three-dimensional (3-D) abrasive article 1. In each repetition, the selected area 180, 181 of loose powder particles 210 may be independently selected, that is, the loose powder particles 210 to be bonded together may be in the same selected area 180, 181 as, or different from, the selected area 180, 181 in previously deposited layers 120.

According to the present disclosure, the focused beam 170 treats a first selected area 180 of the layer 138 of loose powder particles such as to deposit a first amount of energy per unit area to bond powder particles 210 in that first selected area 180 together. The focused beam 170 is guided by mirror system 150 such as to treat the first selected area 180 of the layer 138 of loose powder particles 210 sequentially in a pattern of lines or dots. The power of the focused beam 170 is adjusted such that it deposits a first amount of energy per unit area in the layer 138 of loose powder particles 210 in locations in which the focused beam 170 treats the layer 138. In a location in which the focused beam 170 treats the layer 138, the metallic binder particles 40 thereby melt, flow, bond and re solidify to form a pattern of bonded powder particles in a solid layer 120, while the abrasive particles 30 remain solid, due to the first amount of energy being low enough to not vaporize abrasive particles 30 in the first selected areas 180. The solid layer 120 is thin (typically a few tens of micrometers) and generally not continuous, but comprises open areas of unbonded particles, and solid areas of bonded particles. By creating one solid layer 120 on top of the previous solid layer 120 the metal bond abrasive article is built up layer by layer from the solid areas of each layer 120. The first selected area 180 eventually forms the radially outer portion 80 of the metal bond abrasive article 1 described in the context of Figures 1 and 2.

After treating the first selected area 180 of the layer 138 of loose powder particles 210, the power of the focused beam 170 is increased. The higher-power focused beam 170 then treats a second selected area 181 of the layer 138 of loose powder particles such as to deposit a second, larger amount of energy per unit area to bond powder particles 210 in that second selected area 181 together. The higher-power focused beam 170 is guided such as to treat the second selected area 181 of the layer 138 of loose powder particles 210 sequentially in a pattern of lines or dots. The power of the focused beam 170 is increased such that it deposits a second amount of energy per unit area in the layer 138 of loose powder particles 210 in locations in which the higher-power focused beam 170 treats the layer 138. In a location in which the higher-power focused beam 170 treats the layer 138, the metallic binder particles 40 thereby melt, flow, bond and re-solidify to form a pattern of bonded powder particles in a solid layer 120, and a percentage of the abrasive particles 30 are vaporized and disappear, due to the second amount of energy being high enough to vaporize at least a minimum percentage of the abrasive particles 30 in the second selected area 181. The second selected area 181 eventually forms the radially inner portion 90 of the metal bond abrasive article 1 described in the context of Figures 1 and 2.

Obviously the treatment of the second selected area 181 can be done before or after the treatment of the first selected area 180. It is also contemplated that instead of using the same energy source 160 for the beam 170 and increasing the power of the beam 170 for treating the second selected area 181, a second energy source may be used to generate a second beam of higher power than the first beam, and a second mirror system may guide the second beam to treat the second selected area 181, at the same time that the first beam 170 treats the first selected area 180 of the same layer 138 of powder particles 210. This may speed up the manufacturing process.

Additive manufacturing equipment suitable for practicing the present disclosure is commercially available, for example, from ReaLizer GmbH of DMG Mori (Borchen, Germany) or from EOS GmbH Electro Optical Systems (Krailling, Germany). The metal bond abrasive article 1 comprises the bonded powder particles and remaining loose powder particles 210. Once sufficient repetitions have been carried out to form the metal bond abrasive article, i.e. sufficient layers 120 have been formed sequentially one over the other, the article 1 is preferably separated from substantially all (e.g., at least 85 percent, at least 90 percent, preferably at least 95 percent, and more preferably at least 99 percent) of the remaining loose powder particles, although this is not a requirement. If desired, multiple particle reservoirs each containing a different powder may be used. Likewise, multiple different focused beams may be used, either from a common energy source or, preferably, through separate energy sources.