Chromium-containing coating, a method for its production and a coated object

Abstract

The invention relates to a chromium-based coating comprising at least one layer rich in crystalline phase or phases of nickel (Ni) and/or Ni compounds, and at least one layer rich in crystal-line phase or phases of chromium (Cr) and/or Cr compounds, Cr being electroplated from a trivalent chromium bath. The coating is characterized in that the it further comprises one or more crystalline phases of chromium-nickel-phosphorus (CrNiP), which CrNiP phase has been produced by heat treating a coating comprising at least one layer of nickel-phosphorus (NiP) and at least one layer of Cr. The invention also relates to a method for producing a chromiumbased coating and to a coated object.

Claims

1. A chromium-based coating comprising a plurality of layers including at least one layer rich in crystalline phase or phases of nickel (Ni) and/or Ni compounds, and at least one layer rich in crystalline phase or phases of chromium (Cr) and/or Cr compounds, Cr being electroplated from a trivalent chromium bath, characterized in that the chromium-based coating further comprises one or more crystalline phases of chromium-nickel-phosphorus (CrNiP) produced by heat treating a coating comprising at least one layer of nickel-phosphorus (NiP) and at least one layer of crystalline Cr, wherein at least one of the plurality of layers of the chromium-based coating is a multiphase layer and comprises, in addition to crystalline Cr, at least chromium carbide, and wherein the hardness of the chromium-based coating is at least 1,500 HV.sub.0.005, on a Vickers microhardness scale.

2. A chromium-based coating according to claim 1, wherein the one or more crystalline phases of CrNiP form an interface layer between the at least one layer rich in crystalline phase or phases of Ni and/or Ni compounds and the at least one layer rich in crystalline phase or phases Cr and/or Cr compounds.

3. A chromium-based coating according to claim 1, wherein the chromium-based coating comprises at least two layers rich in crystalline phase or phases of Ni and/or Ni compounds and at least two layers rich in crystalline phase or phases of Cr and/or Cr compounds.

4. A chromium-based coating according to claim 1, wherein at least one of the layers rich in crystalline phase or phases of Ni and/or Ni compounds comprises a crystalline Ni.sub.3P phase.

5. A chromium-based coating according to claim 1, wherein the one or more crystalline phases of CrNiP are components of the at least one multiphase layer.

6. A chromium-based coating according to claim 1, wherein the multiphase layer further comprises at least one of the following: crystalline CrNiP, crystalline CrNi, crystalline Ni, chromium oxide, or a combination thereof.

7. A chromium-based coating according to claim 1, wherein a layer closest to a surface of the coating comprises crystalline Cr.

8. A chromium-based coating according to claim 1, wherein a layer closest to a surface of the chromium-based coating comprises NiP or crystalline Ni.sub.3P.

9. A chromium-based coating according to claim 1, wherein the atomic ratio of the one or more crystalline phases of CrNiP is Cr.sub.10.08Ni.sub.1.92P.sub.7, Cr.sub.0.75Ni.sub.0.25P, Cr.sub.1N.sub.1P.sub.1, Cr.sub.2.4Ni.sub.0.6P, Cr.sub.0.65Ni.sub.0.35P.sub.0.10 or Cr.sub.1.2Ni.sub.0.8P or any combination thereof.

10. A chromium-based coating according to claim 1, wherein the one or more crystalline phases of CrNiP comprises tetragonal CrNiP and/or orthohrombic CrNiP.

11. A chromium-based coating according to claim 1, wherein the thickness of the at least one layer of crystalline Cr is 0.05-20 m.

12. A chromium-based coating according to claim 1, wherein the thickness of the chromium-based coating is 0.5-200 m.

13. A chromium-based coating according to claim 1, wherein the hardness of the chromium-based coating is at least 2,000 HV.sub.0.005 on a Vickers microhardness scale.

14. A chromium-based coating according to claim 1, wherein the Taber index of the chromium-based coating measured by the Taber abrasion test according to ISO 9352 is below 2.

15. A coated object, characterized in that it comprises a chromium-based coating according to claim 1.

16. A coated object according to claim 15, wherein the coated object is a gas turbine, shock absorber, hydraulic cylinder, linked pin, a ball valve or an engine valve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate embodiments of the invention and together with the description help to explain the principles of the invention. In the drawings:

(2) FIG. 1 depicts a part of the XRD spectrum of an embodiment of a coating according to the present disclosure.

(3) FIG. 2 depicts a part of the XRD spectrum of another embodiment of a coating according to the present disclosure.

(4) FIG. 3A depicts a SEM image of the coating presented in FIG. 2

(5) FIG. 3B is an EDS spectrum of a coating of FIG. 2.

(6) FIG. 4 depicts the results of a bending test of a coated object according to the present disclosure.

(7) FIG. 5 depicts the results of an adhesion test of a coated object according to the present disclosure.

(8) FIG. 6 shows the surface structure of a coating with different times between heating and cooling of an object.

(9) FIG. 7 displays a cross-section view of an ion-etched coating according to the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

(10) Reference will now be made in detail to the embodiments of the present invention, an example of which is illustrated in the accompanying drawings.

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

Example 1Preparation of a Chromium-Containing Coating

(12) A steel object was coated with a coating according to the present disclosure. A nickel strike layer was first deposited on the steel substrate (step i)) Then, a 3 m thick NiP layer was chemically deposited on the object (step a)), after which a 5 m thick Cr layer was electroplated on it (step b)). This was followed by a brief acid treatment with 30% (w/w) HCl and deposition of a 1 m Ni strike layer (step i)). After this, steps a) and b) were repeated. Then, the object was heated in a furnace at 850 C. for 30 minutes to amend the mechanical and physical properties of the coating and to produce a CrNiP phase (step c).

(13) X-ray diffraction spectra (XRD) of the chromium-containing coating were measured to get information about the crystalline structure of the coating after heat treatment. Most crystalline materials have unique X-ray diffraction patterns that can be used to differentiate between materials. The peaks of the XRD spectrum were identified by comparing the measured spectrum with the X-ray diffraction patterns of the elements known to be contained in the coating.

(14) Sometimes the top-most layer of a coating to be analyzed can be too thick for performing an XRD analysis directly. In such a case, it is necessary to thin the top-most layer of the coating by, for example, grinding. Thinning methods are known to a skilled person that do not heat the sample so that the properties of the coating would change.

(15) FIG. 1 depicts a portion of the 2-theta XRD spectrum of the coating prepared above after heat treatment. The peaks present in the XRD spectrum of FIG. 1 indicate the presence of crystalline isovite (Cr.sub.23C.sub.6) (denoted with letter A), CrNiP (Cr.sub.2.4Ni.sub.0.6P) (denoted with letter B), metallic chromium (denoted with letter C) and eskolaite (Cr.sub.2O.sub.3) (denoted with letter D). The crystal structure of the CrNiP phase in this embodiment was tetragonal.

Example 2Preparation of a Chromium-Containing Coating

(16) A steel object (in this case, a shock absorber) was coated with a coating according to the present disclosure. First, a 5 m thick NiP layer was chemically deposited on the object (step a)), after which a 7 m thick Cr layer was electroplated on it (step b)). This was followed by 1-2-second acid treatment with 30% (w/w) HCl and the deposition of a 1 m Ni strike layer (current density 2-5 A/dm.sup.2, pH 1.6) (step i)), after which steps a) and b) were repeated. After this, the object was pre-heated at 400 C. with heat pulsing, which in this case was induction heating. After preheating the object was quenched with cooling liquid. The second heat treatment was again performed through induction heating, now at 750-800 C. and quenched with cooling liquid. The pre-heating and the second heat treatment formed step c) of the method according to the present disclosure.

(17) FIG. 2 depicts a portion of the 2 XRD spectrum of the coating prepared above after heat treatment. Also a blow-up image of a portion of the spectra is depicted. In this embodiment, metallic Cr (denoted with letter A), CrNiP (Cr.sub.1.2Ni.sub.0.8P) (denoted with letter B), heptachromium tricarbide (Cr.sub.7C.sub.3) (denoted with letter C) and metallic Ni (denoted with letter D) were present in crystal form.

(18) The morphology of the multilayer coating was observed by scanning electron microscopy (SEM). The composition of the coating was analyzed by energy-dispersive X-ray spectroscopy (EDS) by having an electron beam follow a line in a sample image and generating a plot of the relative proportions of previously identified elements along the spatial gradient.

(19) FIG. 3A depicts the SEM image of the coating prepared by the above method. The vertical arrow indicates the orientation of the coating so that the tip of the arrow points towards the coated substrate. The substrate is visible as the dark gray layer at the bottom of FIG. 3A and the lighter gray layer above it is the layer rich in crystalline phase or phases of nickel (Ni) and/or Ni compounds. Above this layer is a dark grey layer which is a layer rich in crystalline phase or phases of chromium (Cr) and/or Cr compounds. Then the Ni-rich and Cr-rich layers are repeated. The scale bar in the lower right corner of FIG. 3A is 10 m in length and the intensity bar above the micrograph indicates signal strength.

(20) FIG. 3B shows the EDS spectrum of the coating of FIG. 3A. The Cr-rich layer closest to the surface of the coating is on the left and the substrate on the right. The scan coincides with the arrow in FIG. 3A. Prominent layers rich in either Cr or Ni and P, respectively can be identified in FIG. 3B. However, there are interface layers containing all three elements detectible between these layers.

(21) FIG. 4 displays the results of a bending test comparing the coating prepared above to a prior-art hard chromium coating. In the test, the object to be tested rests on two supports that are at a distance of 160 mm from each other. Pressure is exerted on the object at the middle of the supports to induce bending in the object.

(22) On the left, a microscopic image of a hard chromium-coated shock absorber coated with a method known in the art is shown. On the right, a shock absorber coated with the method described above is shown. The images are a 100 magnifications of the surface of the coating from the side that is distal to the exerted pressure, i.e. the results of tensile stress on the coating are displayed. The thickness of the coating in both cases was 15 m and the bending of the compared objects equal.

(23) The difference between the coatings is clearly visible: the prior art coating exhibits extensive delamination (i.e cracking and scaling), which will lead to impairment of the corrosion resistance of the shock absorber when used. The coating according to the present disclosure, however, displays a much lower degree of delamination resulting in better corrosion resistance of the shock absorber. This is indicative of how brittle or tough the coating is. A tough coating, such as the one on the right in FIG. 4 does not break upon bending.

(24) FIG. 5 depicts the results of an adhesion test comparing the coating prepared above to a prior-art chromium coating produced by the use of trivalent chromium. Rockwell HRC hardness test method (also known as the Daimler-Benz adhesion test) was used as the test for adhesion. In this method, a diamond indenter is pressed against the object to be tested and the edges of the indentation left by the indenter are examined for cracks and detachment of the coating from the substrate.

(25) On the left in FIG. 5, a microscopic image of a shock absorber coated with a trivalent chromium coating method and containing a Ni underlayer known in the art is shown. On the right, a shock absorber coated with the method presented above is shown. The images are a 100 magnifications of the surface of the coating. The thickness of the coating in both cases was 15 m.

(26) FIG. 5 displays the mark left by the indenter as a dark circle in the middle of each panel. In the reference shock absorber on the left, the coating severe detachment from the substrate: the substrate around the indentation is exposed. On the right, the coating according to the present disclosure remains attached to the substrate and does not display any cracking. The coating according to the present disclosure thus has better scratching and impact resistant properties.

(27) FIG. 6 shows the surface structure of a coating with different times between heating and cooling of an object. In FIG. 6 on the left, coating according to the present disclosure is depicted, wherein the coating was heated with an induction coil moving along the surface at a speed of 1,500 mm min.sup.1 followed by a cooling liquid loop moving with the same speed 25 mm behind the induction coil. On the right, on the other hand, coating according to the present disclosure is depicted, wherein the distance between the induction coil and the cooling liquid loop was 10 mm while other parameters of the treatment remained the same.

(28) It is evident from FIG. 6 that the surface structure of the coating is influence by the length of time between heating and cooling. On the left, the network of cracks is much denser than on the right. By adjusting the time between the end of the heating and the beginning of the cooling, it is thus possible to change the surface structure of the coating. The surface structure plays a role in, for example, lubricating properties as well as corrosion and wear resistance of the coating, which are thus also adjustable through the method parameters.

(29) FIG. 7 displays a cross-section view of an ion-etched coating according to the present disclosure. The panel on the left is an overview of the coating with the surface of the coating towards the bottom of the figure. The panel on the right is a magnification of the box indicated in the panel on the left. The dark grey layers (A) indicate Cr-rich layers. Cracks are visible in the Cr layers. The light grey layers (B) indicate Ni-rich layers and the mid-grey layer (C) at the top of FIG. 7 is the metal substrate. Interface layers (C) are visible between the mentioned layers. As is evident from FIG. 7, the composition and structure of the interface layers can vary and they can be multiphase layers. These variations are determined by the specifics of the coating method and by the structure and composition of the layers next to the interface layers.

(30) The embodiments of the invention described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment of the invention.

(31) It is obvious to a person skilled in the art that with the advancement of technology, the basic idea of the invention may be implemented in various ways. The invention and its embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.