TECHNICAL FIELD

The present disclosure relates to an object comprising a chromium-based coating on a substrate. The present disclosure further relates to a method for producing an object comprising a chromium-based coating on a substrate.

BACKGROUND

Objects which are utilized in demanding envi ronmental conditions often require e.g. mechanical or chemical protection, so as to prevent the environmen tal conditions from affecting the object. Protection to the object can be realized by applying a coating thereon, i.e. on the substrate. Disclosed are protec tive coatings for various purposes, hard-coatings that protect the substrate from mechanical effects and dif fusion barriers for protection against chemical ef fects. However, further manners to produce hard- coatings in an environmentally friendly manner are needed.

SUMMARY

An object comprising a chromium-based coating on a substrate is disclosed. The chromium is electroplated from an aqueous electroplating bath comprising trivalent chromium cations. The chromium- based coating comprises at least one chromium- containing layer, the chromium-based coating does not contain macro-cracks, wherein a macrocrack is a crack that extends from the outer surface of the chromium- based coating, through the chromium-based coating, to the substrate, the chromium-based coating has a Vickers microhardness value of 800 - 1100 HV, and the chromium-based coating exhibits a critical scratch load value (L _C 2) of at least 60 N in the adhesion test according to ASTM C1624 - 05 (2015; point 11.11.4.4) .

Further is disclosed a method for producing an object comprising a chromium-based coating on a substrate. The method comprises:

- depositing at least one chromium-containing layer on the substrate by subjecting the substrate to at least one electroplating cycle from an aqueous electroplating bath comprising trivalent chromium cat ions, wherein each of the electroplating cycles is carried out at a current density of 150 - 400 A/dm ² for 0.5 - 60 minutes, to produce a chromium-based coating that does not contain macrocracks, wherein a macrocrack is a crack that extends from the outer surface of the chro mium-based coating, through the chromium-based coat ing, to the substrate; and has a Vickers microhardness value of 800 - 1100 HV; and exhibits a critical scratch load value (L _C 2) of at least 60 N in the adhe sion test according to ASTM C1624 - 05 (2015; point 11.11.4.4).

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the embodiments and constitute a part of this specification, illustrate an embodiment. In the drawings:

Fig. 1 discloses a schematical figure of objects comprising a chromium-based coating on a substrate;

Fig. 2 discloses a cross-section view of an image taken by scanning electron microscope (SEM) of a chromium-based coating prepared as disclosed in the current specification and lacking macrocracks; and

Figs. 3a and 3b disclose a cross-section view of an image taken by scanning electron microscope (SEM) of a chromium-based coating comprising macrocracks.

DETAILED DESCRIPTION

The present disclosure relates to an object comprising a chromium-based coating on a substrate. The chromium is electroplated from an aqueous electroplating bath comprising trivalent chromium cations. The chromium-based coating comprises at least one chromium-containing layer, the chromium-based coating does not contain macro-cracks, wherein a macrocrack is a crack that extends from the outer surface of the chromium-based coating, through the chromium-based coating, to the substrate, the chromium-based coating has a Vickers microhardness value of 800 - 1100 HV, and the chromium-based coating exhibits a critical scratch load value (L _C 2) of at least 60 N in the adhesion test according to ASTM Cl624 - 05 (2015; point 11.11.4.4).

The present disclosure further relates to a method for producing an object comprising a chromium- based coating on a substrate. The method comprises:

- depositing at least one chromium-containing layer on the substrate by subjecting the substrate to at least one electroplating cycle from an aqueous electroplating bath comprising trivalent chromium cat ions, wherein each of the electroplating cycles is carried out at a current density of 150 - 400 A/dm ² for 0.5 - 60 minutes, to produce a chromium-based coating that does not contain macrocracks, wherein a macrocrack is a that extends from the outer surface of the chromium- based coating, through the chromium-based coating, to the substrate, and has a Vickers microhardness value of 800 - 1100 HV, and exhibits a critical scratch load value (L _C 2) of at least 60 N in the adhesion test ac cording to ASTM C1624 - 05 (2015; point 11.11.4.4) . In one embodiment, the electroplating is direct current (DC) electroplating.

The inventors surprisingly found out that it is possible to produce a chromium-based coating having a sufficient hardness value and adhesion to the sub strate while lacking the presence of macrocracks. A macrocracks is a large-scale crack in a material. The expression "macrocrack" should be understood in this specification, unless otherwise stated, as referring to a crack that extends from the outer surface of the chromium-based coating, through the chromium-based coating, to the substrate. The macrocrack may have a width over 1 ym. The width of the macrocrack being over 1 ym should be considered as referring to the width at any part of the crack. I.e. the width of a macrocrack may vary.

In one embodiment, the method for producing an object comprising a chromium-based coating on a substrate comprises producing the object comprising a chromium-based coating on a substrate as defined in the current specification.

The inventors surprisingly found out that the adhesion of the chromium-based coating to the substrate may be improved or increased by the method as disclosed in the current specification.

The chromium-based coating exhibits a criti cal scratch load value (L _C 2) of at least 60 N in the adhesion test according to ASTM C1624 - 05 (2015; point 11.11.4.4). In the adhesion test the critical scratch load value (L _C 2) is recorded as the normal force at which damage is first observed. I.e. L _C 2 is associated with the start of chipping failure extend ing from the arc tensile cracks, indicating adhesive failure between the coating and the substrate or part or rhe SUDSrrate.

In one embodiment, the chromium-based coating exhibits a critical scratch load value of at least 80 N, or at least 100 N, or at least 120 N, or at least 150 N, in the adhesion test according to ASTM C1624 - 05 (2015; point 11.11.4.4).

In one embodiment, the chromium-based coating does not contain chromium carbide. In one embodiment, the chromium-based coating is not subjected to a heat treatment. In one embodiment, the at least one chromi um-containing layer is not subjected to a heat treat ment. In one embodiment, the method for producing the chromium-based coating is carried out without subject ing the chromium-based coating to a heat treatment. The inventors surprisingly found out that with the method as disclosed in the current specification, it is possible to produce a hard chromium-based coating having a Vickers microhardness value of 800 - 1100 HV without the use of a heat treatment of the chromium- containing layers deposited from the electroplating bath. The expression "heat treatment" should be under stood in this specification, unless otherwise stated, as referring to subjecting the deposited chromium- containing layers or the chromium-based coating to a heat treatment at a temperature of 300 - 1200 °C for a period of time that would result in the formation of chromium carbides in the chromium-based coating. Such a heat treatment may further change the crystalline structure of chromium. I.e. the method for producing the chromium-based coating may comprise the provision that the deposited chromium-containing layers are not subjected to a heat treatment to form a chromium-based coating having a Vickers microhardness value of 800 - 1100 HV. This provision may not, however, exclude e.g. dehydrogenation annealing.

The Vickers microhardness may be determined according to standard ISO 14577-1:2015. In one embodi ment, the chromium-based coating has a Vickers micro hardness value of 900 - 1090 HV, or 910 - 1080 HV, or 950 - 1060 HV. In one embodiment, the chromium-based coating has a thickness of 1 - 500 ym, or 3 - 300 ym, or 5 - 50 ym. In one embodiment, the electroplating cycle is continued until a chromium-containing layer having a thickness of 1 - 120 ym, or 4 - 35 ym, or 2 - 50 ym, is formed. The thickness may be determined by calculating from the cross-section view of an image taken by scanning electron microscope (SEM).

In one embodiment, each of the electroplating cycles is continued for 0.5 - 60 minutes, or 0.5 - 40 minutes, or 0.5 - 30 minutes, or 0.5 - 25 minutes, or 0.5 - 20 minutes, or 1 - 15 minutes, or 5 - 10 minutes.

In one embodiment, the chromium-based coating has a crystal size of 3 - 35 nm, or 12 - 30 nm, or 14 - 25 nm. The crystal size may be determined in the following manner:

Samples are measured with X-ray diffraction (XRD) in a Grazing incidence (GID) geometry. In GID- geometry the X-rays are targeted on the sample with a small incident angle and held constant during the measurement. In this way, the X-rays can be focused on the surface layers of the sample, with the purpose of minimizing the signal from the substrate. The measure- ments are performed on a 2Q angular range of 30°-120°, with increments of 0.075°. A total measurement time for each sample is 1 h. The incident angle of X-rays is 4°. In addition to the samples, a corundum standard (NIST SRM 1976a) was measured with identical setup to measure the instrumental broadening of diffraction peaks. The measurements are performed on a Bruker D8 DISCOVER diffractometer equipped with a Cu K X-ray source. The X-rays are parallelized with a Gobel mir ror, and are limited on the primary side with a 1 mm slit. An equatorial soller slit of 0.2° is used on the secondary side. The phases from the samples are iden tified from the measured diffractograms with DIF- FRAC.EVA 3.1 software utilizing PDF-2 2015 database. The crystal sizes and lattice parameters are deter mined from the samples by full profile fitting per formed on TOPAS 4.2 software. The instrumental broad ening is determined from the measurement of the corun dum standard. The crystal sizes are calculated using the Schem er equation [see Patterson, A. (1939). "The Schem er Formula for X-Ray Particle Size Determina tion". Phys. Rev. 56 (10): 978-982.], where the peak widths are determined with the integral breadth method [see Scardi, P., Leoni, M., Delhez, R. (2004), "Line broadening analysis using integral breadth methods: A critica1 review ". J. App1. Crysta11ogr. 37: 381-390].

The obtained values for lattice parameters are com pared to literature values. The difference in measured values and literature values suggest the presence of residual stress within the coating.

In one embodiment, the chromium-based coating is characterized by an X-ray powder diffraction pat tern containing specific peaks at 44° and 79° 2theta (2Q). In one embodiment, the chromium-based coating is characterized by an X-ray powder diffraction pattern containing specific peaks at 44.5°, 64.7°, 81.8°,

98.2°, and 115.3° 2theta (2Q).

The chromium-based coating may comprise 87 - 99 weight-%, or 92 - 97 weight-% of chromium. The chromium-based coating may comprises 0.3 - 5 weight-%, or 1.0 - 3.0 weight-% of carbon. The chromium-based coating may also comprise nickel and/or iron. The chromium-based coating may comprise also other ele ments. The chromium-based coating may in addition com prise oxygen and/or nitrogen.

As is clear to the skilled person, the chro mium-based coating may in addition to the materials presented above contain minor amounts of residual ele ments and/or compounds originating from manufacturing process, such as the electroplating process. Examples of such further elements are copper (Cu), zinc (Zn), and any compounds including the same.

The amounts of different elements, such a chromium, iron, nickel, etc., in the chromium-based coating may be measured and determined with an XRF an alyzer. The amount of carbon in the chromium-based coating may be measure and determined with an infrared (IR) detector. An example of such a detector is the Leco C230 carbon detector.

As is clear to the skilled person, the total amount of the different elements in the chromium-based coating may not exceed 100 weight-%. The amount in weight-% of the different elements in the chromium- based coating may vary between the given ranges.

In one embodiment, the object is a gas tur bine, shock absorber, hydraulic cylinder, linked pin, joint pin, a bush ring, a round rod, a valve, a ball valve, or an engine valve.

Some methods, in order to achieve hard chro mium-based coatings, may have required the use of at least one heat treatment of the deposited chromium- containing layer(s) or the chromium-based coating at a temperature of 300 - 1200 °C, when using an aqueous electroplating bath in which chromium is present sub stantially only in the trivalent form. By omitting this kind of heat treatment, one may be able to form a chromium-based coating that essentially lacks chromium carbides. The term "chromium carbide" is herein to be understood to include all the chemical compositions of chromium carbide. Examples of chromium carbides that may be present in the first layer are Cr ₃ C2, Cr ₇ C3, Cr2 ₃ C6, or any combination of these. Such chromium car bides are usually formed into the chromium-based coat ing when the chromium-containing layer(s) deposited on a substrate by electroplating from a trivalent chromi um bath is subjected to at least one heat treatment at the temperature of 300 - 1200 °C. In this specification, unless otherwise stated, the terms "electroplating", "electrolytic plating" and "electrodeposition" are to be understood as synonyms. By depositing a chromium-containing layer on the substrate, is herein meant depositing a layer directly on the substrate, or at a later stage on a previously deposited chromium-containing layer, to be coated. In the present disclosure, the chromium- containing layer (s) may be deposited through electroplating from an aqueous electroplating bath comprising trivalent chromium cations. In this connection, the wording electroplating "from an aqueous electroplating bath comprising trivalent chromium cations" is used to define a process step in which the deposition is taking place from an electrolytic bath in which chromium is present substantially only in the trivalent form.

In one embodiment, the electroplating cycle is carried out while keeping the temperature of the aqueous electroplating bath at 50 - 70 °C, or 55 - 65 °C, or 58 - 62 °C. The rather low temperature of the aqueous electroplating bath used in the electroplating cycle has the added utility of improving the adhesion of the chromium-containing layer and thus the whole formed chromium-based coating to the substrate.

In one embodiment, the electroplating cycle is carried out at a current density of 150 - 300 A/dm ² , or 170 - 300 A/dm ² , or 200 - 250 A/dm ² . The inventors surprisingly found out that when the chromium-based coating is formed by using a rather high current density, a chromium-based coating lacking macrocracks may be produced. Using an aqueous electroplating bath of trivalent chromium cations may result in that macrocracks are formed in the coating. The inventors surprisingly found out that these macrocracks may be prevented by using the higher current density in the electroplating cycle. Each of the at least one electroplating cy cles may be separated from another electroplating cy cle in time so as to form chromium-containing layers arranged one upon the other. In one embodiment, each of the electroplating cycles is separated from one an other in time by stopping the electroplating process for a predetermined period of time. Each of the elec troplating cycles is separated from another electro plating cycle by at least 1 second, or at least 10 seconds, or at least 30 seconds, or at least 1 minute, or at least 5 minutes, or at least 10 minutes. In one embodiment, each of the electroplating cycles is sepa rated from another electroplating cycle by 0.1 milli seconds - 3 minutes, or 1 second - 60 seconds, or 10 - 30 seconds. In one embodiment, each of the electro plating cycles is separated from another electroplat ing cycle by 0.5 - 10 minutes, or 2 - 8 minutes, or 3 - 7 minutes.

Different electroplating cycles may be separated from each other by stopping the current to pass through the aqueous electroplating bath. The substrate to be subjected to the electroplating may be removed from the aqueous electroplating bath for a certain period of time and then put back into the bath for continued electroplating. The substrate to be subjected to electroplating may be removed from one trivalent chromium bath for a certain period of time and placed in another trivalent chromium bath for the sequential electroplating cycle to take place.

In one embodiment, the aqueous electroplating bath used in a first electroplating cycle is different from the aqueous electroplating bath used in the following electroplating cycle. In one embodiment, the aqueous electroplating bath used in the different electroplating cycles is the one and the same.

The aqueous electroplating bath comprising trivalent chromium cations may in addition to trivalent chromium cations comprise carboxylate ions. The bath may comprise trivalent chromium cations in an amount of 0.12 - 0.3 mol/1, or 0.13 - 0.24 mol/1, or 0.17 - 0.21 mol/1. The bath may comprise carboxylate ions in an amount of 1.22 - 7.4 mol/1, or 2.0 - 6.0 mol/1, or 2.3 - 3.2 mol/1. The molar ratio of trivalent chromium cations to the carboxylate ions may be 0.015 - 0.099, or 0.015 - 0.09, or 0.03 - 0.08, or 0.065 - 0.075 in the aqueous electroplating bath. Any soluble trivalent chromium salt(s) may be used as the source of the trivalent chromium cations. Examples of such trivalent chromium salts are potassi um chromium sulfate, chromium (III)acetate, and chromi um (III)chloride. The source of carboxylate ions may be a car boxylic acid, such as formic acid, acetic acid, or citric acid, or any combination thereof.

The aqueous electroplating bath may further contain iron cations and/or nickel cations. The aque- ous electroplating bath may comprise iron cations in an amount of 0.18 - 3.6 mmol/1, or 0.23 - 0.4 mmol/1. The aqueous electroplating bath may comprise nickel cations in an amount of 0.0 - 2.56 mmol/1, or 0.53 - 1.2 mmol/1. The aqueous electroplating bath may com- prise iron cations and nickel cations in an amount of 0.18 - 6.16 mmo1/1, or 0.76 - 1.6 mmo1/1.

The aqueous electroplating bath may comprise bromide ions in an amount of 0.15 - 0.3 mol/1, or 0.21 - 0.25 mol/1. The source of the bromide ions may be selected from a group consisting of potassium bromide, sodium bromide, ammonium bromide, and any combination or mixture thereof.

The aqueous electroplating bath may comprise ammonium ions in an amount of 2 - 10 mol/1, or 2.5 - 6 mol/1, or 3 - 4 mol/1, or 0.18 - 1.5 mol/1, or 0.45 - 1.12 mol/1. The source of the ammonium ions may be se lected from a group consisting of ammonium chloride, ammonium sulfate, ammonium formate, ammonium acetate, and any combination or mixture thereof.

The pH of the aqueous electroplating bath may be 2 - 6, or 3 - 5.5, or 4.5 - 5, or 4.1 - 5. The pH may be adjusted by including a base in the aqueous electroplating bath when needed. Ammonium hydroxide, sodium hydroxide, and potassium hydroxide may be men tioned as examples of bases that may be used for ad justing the pH of the aqueous electroplating bath. The aqueous electroplating bath may comprise a base in an amount of 0.5 - 3.1 mol/1, or 1.4 - 1.8 mol/1.

The conductivity of the aqueous electroplating bath may be 160 - 400 mS/cm, or 200 - 350 mS/cm, or 250 - 300 mS/cm. The conductivity of the aqueous electroplating bath may be adjusted with the use of e.g. different salts for conductivity. Ammonium chloride, potassium chloride, and sodium chloride can be mentioned as examples of salts that may be used to adjust the conductivity. The conductivity may be determined e.g. in compliance with standard EN 27888 (water quality; determination of electrical conductivity (ISO 7888:1985)).

The method and the chromium-based coating as disclosed in the current specification are well suited for protecting metal substrates from corrosion. In one embodiment, the corrosion resistance of the object is at least 24 h, or at least 48 h, or at least 96 h, or at least 168 h, or at least 240 h, or at least 480 h. The corrosion resistance can be determined in accordance with standard EN ISO 9227 NSS (neutral salt spray) rating 9 or 10 (2017).

By a "substrate" is herein meant any compo nent or body on which the chromium-based coating as disclosed in the current specification is coated on. Generally, the chromium-based coating as disclosed in the current specification can be used on variable sub strates. In one embodiment, the substrate comprises or consists of metal, a combination of metals, or a metal alloy. In one embodiment, the substrate is made of steel, copper, nickel, iron, or any combination there of. The substrate can be made of ceramic material. The substrate does not need to be homogenous material. In other words, the substrate may be heterogeneous mate rial. The substrate can be layered. For example, the substrate can be a steel object coated by a layer of nickel, or nickel phosphorus alloy (Ni-P). In one em bodiment, the substrate is a cutting tool, for example a cutting blade. In one embodiment, the substrate is a cutting tool comprising metal.

In one embodiment, the object comprising a chromium-based coating on a substrate does not comprise a layer of nickel. In one embodiment, the chromium-based coating does not comprise a layer of nickel. In one embodiment, the substrate does not comprise a layer of nickel.

The object disclosed in the current specification has the added utility of lacking the presence of macrocracks. I.e. the chromium-based coating contains essentially no macrocracks.

The object disclosed in the current specifi cation has the added utility of being well suited for applications wherein hardness of the object is rele vant. The materials of the chromium-based coating have the added utility of providing the substrate a hard ness suitable for specific applications requiring high durability of the object.

The object disclosed in the current specifi cation has the added utility of the chromium-based coating exhibiting good adhesion to the substrate as a result of the production method as disclosed in the current specification.

The chromium-based coating has the added utility of protecting the underlying substrate from effects caused by the interaction with the environment during use. The chromium-based coating has the added utility of providing a good corrosion resistance. The chromium-based coating further has the added utility of being formed from trivalent chromium, whereby the environmental impact is less than when using hexava- lent chromium. Further, the method as disclosed in the current specification has the added utility of being a safer production method for a chromium-based coating than if hexavalent chromium is used. Further, being able to omit the heat treatment of the chromium- containing layer while still providing a chromium- based coating with a high Vickers microhardness value and good adhesion of the chromium-based coating on the substrate, has the added utility of simplifying the production method and thus beneficially affects the production costs.

EXAMPLES

Reference will now be made in detail to various embodiments, examples of which are illustrated in the accompanying drawings.

The description below discloses some embodiments in such a detail that a person skilled in the art is able to utilize the embodiments based on the disclosure. Not all steps or features of the embodiments are discussed in detail, as many of the steps or features will be obvious for the person skilled in the art based on this specification.

Fig. 1 discloses on the left-hand side a schematical figure of an object comprising a chromium- based coating on a substrate, wherein the chromium- based coating comprises macrocracks and on the right- hand side a schematical figure of an object comprising a chromium-based coating on a substrate, wherein the chromium-based coating does not comprise macrocracks. Example 1 - Preparing a chromium-based coating on a substrate

In this example different objects, each comprising a chromium-based coating on a substrate, were prepared.

Firstly, the substrates were pre-treated by cleaning the metal substrates, i.e. CK45 steel substrates, and providing thereon by electroplating and as a part of the substrate a nickel layer having a thickness of about 3 - 4 pm. Thereafter the substrates were rinsed with water after which the chromium-based coating was formed on the substrate.

The aqueous electroplating bath comprised the following:

The aqueous electroplating bath was subjected to a normal initial plating, after which it was ready for use.

Then a chromium-based coating was deposited on the substrate by subjecting the substrate to an electroplating cycle. The electroplating cycle was carried out as follows:

Current density: 220 A/dm ² Time: 6 minutes Temperature of the bath: 55 °C

The properties of the chromium-based coating were measured according to measurement methods presented above in the current specification and the results are presented below:

Thickness: 25 ym

Vickers microhardness value: 880 HV Crystal size: 5 nm

Macrocracks No

For comparison, comparative example was prepared in anotherwise similar manner as above described but with carrying out the electroplating as follows:

Current density: 60 A/dm ²

Time: 20 minutes Temperature of the bath: 55 °C

The properties of the chromium-based coating were measured according to measurement methods presented above in the current specification and the results are presented below:

Thickness: 25 ym

Vickers microhardness value: 800 HV

Crystal size: 4 nm Macrocracks Yes

As can be from Fig. 2, the chromium-based coating prepared in example 1 contains no macrocracks, while the chromium-based coating of the comparative example (see Fig. 3a and 3b) clearly contains large macrocracks that extend through the coating to the surface of the substrate. It is obvious to a person skilled in the art that with the advancement of technology, the basic idea may be implemented in various ways. The embodiments are thus not limited to the examples described above; instead, they may vary within the scope of the claims.

The embodiments described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment. An object, or a method, disclosed herein, may comprise at least one of the embodiments described hereinbefore. It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those items. The term

"comprising" is used in this specification to mean including the feature(s) or act(s) followed thereafter, without excluding the presence of one or more additional features or acts.