SURFACE TREATMENT FOR ZIRCONIA BASED MATERIALS

The invention is directed to a method for improving the bonding strength between zirconia -based materials and other materials, and to zirconia-based materials obtainable by this method. The materials of the present invention have improved surface properties.

Fully or partially stabilized zirconia, and in particular zirconia stabilized with yttrium, is a material that is widely used in the preparation of ceramic compositions. This material is very promising and finds increasing use as replacement for metal alloys, such as in orthopedic devices, dental prostheses and the like. Stabilized zirconia has amongst others excellent mechanical and chemical properties.

However, the inert outer surface of stabilized zirconia causes difficulties, because hardly any material can bond to it. Therefore, the bond strengths between zirconia and other materials, such as adhesive resins, ceramics, or bone structures, are in general relatively weak. In the case of implants, unsatisfactory bone integration with the surface of the implant may result. Moreover, most known bonds between zirconia and other materials deteriorate with time.

Another problem using zirconia-based materials in e.g. dentistry is that the bond strength with veneering ceramics is weak. Veneering ceramics are often used on top of the zirconia-based material in order to obtain an aesthetically nice restoration. However, the weak bond strength between the zirconia-based materials and the veneering ceramics often results in delamination of the veneering ceramics (Dent Mater. 2005, 21, 984-991).

It has been proposed to improve the bonding strength of yttrium oxide partially stabilized zirconia (YPSZ) by using certain resin cements (J. Adhesion Dent. 2000, 2, 139-146). However, for clinical use with YPSZ only one chemically cured resin modified with phosphate ester monomer (10-methacryloyloxydecyl dihydrogenphosphate) was recommended.

Another well-known method for enhancing the bond strength to ceramic structures is by surface coating or conditioning. However, the homogeneous structure of zirconia is hard to acid etch and it was found that zirconia based materials do not respond successfully to common silica coating silanation procedures.

US-A-2002/0 006 532 describes a dental system including yttrium doped tetragonal zirconia polycrystals (YTZP) ceramic. Resin is bound to the bonding surface of the YTZP ceramic, which has a grain size of less than 0.6 μm. The described zirconia has a glassy phase which is chemically eliminated from the bonding surface of the YTZP by chemical treatment with hydrofluoric acid. This treatment with hydrofluoric acid causes chemical dissolution of the glassy phase involving weight loss. This material can not be regarded as glass free polycrystalline ceramic and has significantly lower mechanical properties compared to YTZP materials. Examples of these materials are used in the dental field, e.g. Vita In-Ceram fabricated by Vita Zahnfabrik, Gmbh & Co, Germany.

WO-A-2005/070322 describes a method for preparing an inorganic- inorganic composite comprising preheating an oxide ceramic for example between 600 and 1300 ⁰ C, manufacturing of a crystalline oxide ceramic piece, applying an impregnating compound and heating the composite for example between 1000 and 1600 ⁰ C. The impregnating compound can be the precursor of a non-metallic inorganic phase, an amorphous glass phase, a hydrolysable compound of a metal, or a metal alcoholate. This method depends on adding a removable phase to the starting powder of zirconia. After sintering, this removable phase is either burnt off or chemically dissolved, leaving behind structural porosities. This method not only complicates the fabrication procedure but results in including structural defects in zirconia that significantly weaken the structure.

Object of the invention is to provide a method by which stabilized zirconia material can bond with other materials with enhanced bonding strength.

A further object of the invention is to provide a stabilized zirconia material that does not require a specific resin cement or specific adhesive for bonding with other materials.

Another object of the invention is to provide a stabilized zirconia material that has long-term stable bond strength with other materials.

One or more of these objects have been met by the method of the invention for increasing the bond strength between at least partially stabilized zirconia and another material, wherein said at least partially stabilized zirconia comprises Zrθ2 and at least one stabilizing compound, which method comprises the successive steps of: applying a conditioning agent to said at least partially stabilized zirconia; heating said at least partially stabilized zirconia to a temperature above the glass transition temperature of said conditioning agent; cooling said at least partially stabilized zirconia; and removing substantially all of the conditioning agent from said at ' least partially stabilized zirconia.

The invention further provides a stabilized zirconia material which is a surface modified stabilized zirconia comprising Zrθ2 and at least one stabilizing compound, and which is obtainable by the above-mentioned method.

It has been found that the stabilized zirconia according to the invention has an increased bond strength, which is stable with time.

The term "conditioning agent" as used in this application refers to any material that is applied to the surface of a substrate material to change its chemical or physical nature. In particular, the conditioning agents suitable for

the present invention are able to change the surface of zirconia from smooth surface to a retentive active surface.

The term "etching" as used in this application refers to changing the surface architecture of a material from smooth to rough by a displacement of the surface molecules, grains, and crystals without using an acidic medium and without loss of weight or dissolution of substrate material. Thus, etching as used in this application is not meant to refer to chemical dissolution involving weight loss, but rather to a restructuring of surface molecules, grains, and crystals.

Pure Zrθ2 has a monoclinic crystal structure at room temperature. Upon heating it may undergo a phase transition via a tetragonal crystal structure or to a cubic crystal structure at increasing temperatures. The volume expansion caused by the reversed transition (cubic → tetragonal → monoclinic transformation) upon cooling induces very large stresses, and will cause pure Zrθ2 to crack upon cooling from high temperatures. Several different oxides are added to zirconia to stabilize the tetragonal and/or cubic phases. Suitable stabilizers are for instance MgO, CaO, Y2O3, Ceθ2, AI2O3, SC2O3 and Yb2θ3. Depending on the stabilizer used, partially stabilized zirconia (PSZ) or tetragonal zirconia polycrystals (TZP) may be obtained. The amount of stabilizer may vary from 0.05 to 10 mol%, more preferably between 0.1 and 5 mol%. Most preferred for use in the present invention is yttrium stabilized TZP having 1-4 mol%, more preferably about 3 mol% of Y2O3.

In some cases, the tetragonal phase can be metastable. If sufficient quantities of the metastable tetragonal phase are present, then an applied stress, magnified by the stress concentration at a crack tip, can cause the tetragonal phase to convert to monoclinic, with the associated volume expansion. This phase transformation can put the crack into compression, retarding its growth, and enhancing the fracture toughness. This mechanism is known as transformation toughening, and significantly extends the reliability and lifetime of products made with stabilized zirconia.

A special case of zirconia is that of tetragonal zirconia polycrystalline or TZP, which is indicative of polycrystalline zirconia composed of only the metastable tetragonal phase. In TZP the use of extremely fine initial powders, and the application of low sintering temperatures, achieves an extremely fine-grained microstructure. Due to its extremely fine microstructure (grain size typically < 1-3 μm) and the metastable tetragonal structure, this material is characterized by extraordinary high mechanical strength, possibly even exceeding 1500 MPa under flexure.

Because of its excellent properties tetragonal zirconia polycrystalline that is partially stabilized with yttrium oxide, is widely used. It should be noted that varieties may exist among these materials, even when these materials have the same chemical composition. Differences in e.g. strength and translucency may occur, for instance as a result of the chosen powder type and the production conditions. In dentistry, typically yttrium stabilized TZP is used with 2-3 mol% OfY ₂ O ₃ . This is generally referred to as 3Y-TZP.

Surprisingly, it has now been found that the bond strength of stabilized zirconia with other materials can be significantly improved by modifying the surface of the zirconia crystals, in a way that the surface becomes nanostructured. This results in increased surface energy and thus in an increased possibility to establish nanoretention with many materials. The stabilized zirconia according to the invention has excellent bond strength and can be used without the need for special surface, chemical, mechanical treatments or any specific bonding agents.

Typically, the zirconia according to the invention is prepared by the following procedure.

Step I: Maturation Heat treatment

The maturation heat treatment is an optional pretreatment. This step is dependent on the chemical structure and physical structure of the used zirconia, especially the crystal sizes and the inter-grain structure. During the

maturation heat treatment the stabilized zirconia crystals are first heated to a temperature above 800 ⁰ C. For most stabilized zirconia materials a temperature in the range of 800 - 1100 ⁰ C is suitable, preferably a range of 900 - 1000 ⁰ C is used. The zirconia crystals are preferably held at this temperature for a certain amount of time to allow maturation of the grain sizes. Typically, the hold time is 1 to 15 minutes, more preferably 8 to 12 minutes.

Without wishing to be bound by theory it is believed that during the hold time of the heating step, the zirconia crystals at the surface undergo a monoclinic-tetragonal phase change. The selected heating temperature is therefore in the range of the tetragonal-monoclinic phase transformation temperature. Although this phase transformation temperature may vary with the type of stabilized zirconia used, it is typically above 800 ⁰ C. The phase transition is characterized by maturation of the grain structure of zirconia into more homogenous and evenly distributed shapes. Independent crystals at grain boundaries tend to join the neighboring grains and thus create nanospaces between the grains and result in a nanostructured surface on both the ultra structured crystal level and on the micro structural grain level. Thus, this heat treatment takes advantage of the dynamic transformation properties of zirconia and uses it to establish three basic structural changes:

A - Maturation of grain sizes, which is obtained through: B - Redistribution of the zirconia crystals at the surface. C - Growth and widening of grain boundary regions

These three structural changes result in changing the bond strength between the zirconia grains and crystals especially at the surface of the material.

Next, the zirconia is cooled. This can for example be a cooling to ambient, but other ways of cooling may also be applied. It is believed that during this cooling the crystals at the surface undergo a reverse phase transition. These phase transitions have the effect that the bonds between the

zirconia grains and crystals at the surface weakens and that the grain boundary regions becomes pre-stressed.

Although the effect on the zirconia surface is already significant after heating and cooling one time, in a preferred embodiment, the heating and cooling are switched several times, e.g. 2-10 times, in order to increase the change in grain size and surface structure if deeper structural changes are required below the surface for a depth of more than 3-5 μm.

The change in surface properties upon the maturation heat treatment can be monitored using scanning electron microscopy (SEM). Figures 1 and 2 show SEM images of a zirconia surface before and after subjecting the zirconia to maturation heat treatment.

Step II: Selective Infiltration Etching

In many cases the maturation heat treatment step can be selectively added or combined with the selective infiltration etching step, in order to shorten the preparation time.

In the selective infiltration etching step, a conditioning agent is applied, followed by a specific activation cycle, which allows the top layer of the zirconia grains and crystals to be etched. The activation cycle comprises the application of a conditioning agent, heating above the glass transition temperature of the conditioning agent, cooling, and washing off the remainder of conditioning agent.

The conditioning agent is the material that is responsible for the change of the zirconia surface from smooth to a very retentive and active. This surface change, which is referred to as etching in the present description and claims, occurs during the specific activation cycle. Adjustment of the activation cycle allows selective etching of zirconia crystals.

Typical conditioning agents, which may be used for the purpose of the invention, are acidulated or balanced non-ionized oxidized mixtures of one or more compounds selected from aluminum, potassium, iron, magnesium,

rubidium, glass powder, silica and the like. Trace elements, which help to reduce both time and temperature of this treatment, may be present as well. The conditioning agent preferably comprises an inorganic oxide selected from one or more of the following components in the indicated amounts (weight percentage based on the total weight of the conditioning agent): silica (10-60 wt.%, preferably 20-50 wt.%, e.g. about 30 wt.%), alumina (1-25 wt.%, preferably 2-20wt.%, e.g. about 8 wt.%), potassium (1-18 wt.%, preferably 2-10 wt.%, e.g. about 3 wt.%), rubidium (0-10 wt.%, preferably 0.5-5 wt.%, e.g. about 1 wt.%), titanium (1-22 wt.%, preferably 5-17 wt.%, e.g. about 13 wt.%), zirconia (0-5 wt.%, preferably 0.5-3 wt.%, e.g. about 1 wt.%), and other trace elements where oxygen is a balance. The conditioning agent is preferably applied as a very thin wash layer to fully sintered zirconia and exerts its effects mainly to the surface grains of zirconia, whereas in WO-A-2005/070322 the green state of zirconia is impregnated with a secondary phase that is burnt during sintering or can be removed after sintering resulting in gaps and structural defects.

The conditioning agent is applied on the surface to be treated. Subsequently, the mixture is activated by heat treatment above the glass transition temperature of the conditioning agent and the temperature is kept constant for about 30 seconds or more, preferably 50 seconds or more, depending on the structure of the stabilized zirconia crystals. Then, the treated zirconia is cooled and substantially all of the remaining conditioning agent is removed, preferably by washing the treated zirconia.

Acidulated washing solution can be used to wash out remnants of conditioning agent. The acidulated washing solution can be applied for a time ranging for instance between 30 minutes to 2 hours depending on the surface area requiring treatment. Preferably, this procedure is performed under ultrasonic conditions. If a non- acidulating conditioning agent is used then a more concentrated washing solution can be used and the washing time may be doubled. A preferred washing agent in this case is a low concentration,

typically 1-35%, acid mixture containing hydrofluoric acid. Removal of the conditioning agent exposes the created surface roughness which is ready for bonding to other materials. It is therefore preferable to remove substantially all of the conditioning agent, and even more preferably to remove all of the conditioning agent and to completely exposed the treated zirconia surface.

After treatment with conditioning agent the surface energy of the treated surface is very high and the surface of the zirconia is very reactive. In addition, the structure of zirconia crystals at the surface becomes well architectured for establishing nanomechanical retention with many materials. The treated zirconia may be removed from the washing bath and kept in a concentrated alcohol solution. Subsequently a bonding preparation procedure can be performed. Figure 3 shows a SEM image of zirconia after complete treatment with both maturation heat treatments and selective infiltration etching in a combined treatment cycle.

The untreated stabilized zirconia usually has a surface roughness of 0.02 to 0.03 μm when properly polished. This starting surface roughness can be increased by the method of the invention to a value of at least 0.5 μm for good retention. If required, higher surface roughness can be obtained in thick zirconia samples by prolonging the selective infiltration etching procedure, optionally combined with the maturation heat treatment. In addition to surface roughness, the created nano-spaces between grain boundaries and crystals have a three dimensional distribution that can directly enhance bonding with zirconia as the bonding agent of choice can infiltrate and interlock with these nano-spaces.

The increase in roughness according to the present invention is characterized by the creation of a retentive surface, rather than by surface elevations as obtained during sand blasting procedures. Sand blasting typically results in surface destruction and in a phase shift of the zirconia from tetragonal to monoclinic. In addition, sharp cracks are created. These effects in general weaken the zirconia. The created sharp cracks form a spots that are

liable for future catastrophic fractures thereby reducing the lifespan of the material and causing a direct reduction of the flexure strength of the material.

A surface roughness of more than 6 μm could be detrimental to the specimen strength. Therefore, preferably the surface roughness is less than 6 μm and typically lies in the range of 2-4 μm.

Step III: Bonding Preparation Procedure

After the selective infiltration etching, a bonding preparation procedure can be performed.

In a first embodiment, the bonding preparation procedure comprises the application of a bonding adhesive, which basically comprises a low viscosity epoxy or low viscosity composite resin monomer solutions. For the application of the bonding adhesive, other specific chemical agents such as phosphate monomer are not required. Typical examples of such bonding adhesives are for example a mixture of methyl niethacrylate monomer or low viscosity Bis-Gma (bisphenol A diglycidylether methacrylate) monomer. The zirconia is removed from alcohol and dried. Then, a mixture of preferably non-filled resin is applied and thinned with air. The resin is activated to initiate a polymerization cycle and after polymerization the surface is ready for bonding to any available composite material.

In a second embodiment, which may be optionally combined with the first embodiment, the bonding procedure comprises the application of a liquid mixture of bioactive glass. In particular for medical prostheses or dental implants, a liquid mixture of bioactive glass of choice, for example calcium oxides, phosphosilicate glasses and glass ceramics, may be applied. The bioactive glass may be sintered by heat treatment to form a zirconia surface coated with a thin layer of bioactive material. The coated zirconia may then be mildly roughened and sterilized.

In a third embodiment, which may be optionally combined with the first and/or the second embodiment, the zirconia -based materials that are

subjected to the method of the invention can be veneered with a ceramic material of choice. The improved bond strength, which can be for instance 48 MPa in accordance with the present invention (as compared to, typically 28 MPa for the prior art materials), results in improved strength and lifetime of the layered structure.

The bonding preparation procedure may also comprise the soaking of the treated zirconia in a solution of bioactive glass, human serum or other soft tissue matrix. Hard and soft tissue integrate on the treated surface modified zirconia and thus the zirconia implant becomes accepted and functions within the surrounding tissues.

In accordance with the present invention the bond strength can be improved considerably, from about 25-35 MPa (which are conventional values for e.g. a conventional adhesive resin/YPSZ bond) to bond strength values that are much higher e.g. up to 60 MPa or more, such as 67 MPa.

The stabilized surface modified zirconia with enhanced bond strength has a wide range of applications, for instance prosthetic restorations, implants, solid electrolytes, fuel cells, oxygen sensors, cutting abrasives, electrical insulators, military fields and many other fields.

Example 1

The surface of a 3 Y-TZP material was subjected to the method of the invention. The zirconia was coated with a very thin layer of the conditioning agent composed of silica (30 %), alumina (8 %), potassium (3 %), rubidium (1 %), titanium (13 %), zirconia (1 %) and other trace elements which was then heated to 800 °C at a rate of 30 °C/min under vacuum. The holding time was 60 s after which the zirconia was cooled to room temperature at the same rate. The conditioning agent has a thermal expansion coefficient between 9-11 ppm/°C.

After cooling, the conditioning agent was dissolved in 10 % hydrofluoric acid for 40 min and the zirconia was washed with water, steam cleaned, and dried. SEM analysis was performed, see Figure 3. The zirconia was covered with a thin layer of an adhesive resin and then bonded to dental composite material (Filtek Z250; 3M ESPE, St. Paul, Minn) and light polymerized (Elipar FreeLight 2; 3M ESPE). This system is identified as group 3. The bond strength was determined by a microtensile bond strength test over four time intervals, immediately after bonding, one day, one week and one month after bonding. Two other commercial systems were used as control, identified as group 1 and 2. Not only did the group subjected to the invention demonstrate more than double the bond values compared to the other commercial system but also the bond strength was stable even under storage of water for one month, see Figure 4.

Example 2

Stabilized zirconia was subjected to the method of the invention: maturation heat treatment, Selective Infiltration Etching and Bonding Adhesive Procedure. The sample (the same sample as used in Example 1) was stored under corrosive chemical solution for two months and then subjected to a Microtensile Bond Strength Test to evaluate the bond strength. A value of 67 MPa was recorded which reflects the stability of the bond under corrosive and active environments. Figure 5 shows a SEM photograph of a zirconia - based material according to the invention that has been bonded to an adhesive resin. Subsequently the sample was broken. The picture demonstrates that after breaking the sample, adhesive resin (the 54.5 μm layer) still remains on the surface of zirconia. This reflects the superiority of the bond, because prior art zirconia breaks at the borderline of zirconia and adhesive resin.

Description of Figures.

Figure 1. Normal structure of zirconia showing densely packed zirconia crystals.

Figure 2. Ultra structural micro spacing between zirconia grains after subjecting it to the full heating and cooling cycles as dictated by maturation heat treatment.

Figure 3. Zirconia crystals after final treatment with selective infiltration etching and washing out of the conditioning agent showing less dense surface and inter crystal spacing resulting in highly active and retentive surface.

Figure 4. Bond strength values measured for zirconia subjected to the method of the invention, group 3, compared to comparative systems (group 1 and group 2) over 4 time intervals.

Figure 5. Breaking experiment reflecting a good resin to zirconia bond strength even when stored under corrosive conditions.