GB2454743A

GB2454743A - TiCr binary coating

Info

Publication number: GB2454743A
Application number: GB0722642A
Authority: GB
Inventors: Paul Peeters; Christian Strondl
Original assignee: Hauzer Techno Coating BV
Current assignee: IHI Hauzer Techno Coating BV
Priority date: 2007-11-19
Filing date: 2007-11-19
Publication date: 2009-05-20
Also published as: GB0722642D0; WO2009065545A1

Abstract

The use of a binary TiCr coating 12 as a top layer or an underlayer for corrosion protection on a metallic substrate 10. The substrate can be any one of iron, steel, aluminium or an aluminium alloy, magnesium or a magnesium alloy or brass or copper or Zamak or any die-cast material. The TiCr coating can have a constant composition or can be graded so that the proportion of titanium in the layer increase with distance from the substrate. Further top layers can be formed on the coating. The coating can be formed by CVD or PVD processes and may be further subjected to a thermal treatment such as annealing.

Description

The use of a binary coating comprising first and second different metallic elements The present invention relates to use of a binary coating including a layer of at least first and second different metallic elements in the form of Ti and Cr.

TiCr coatings are known per Se. Thus DE 44 46 985 Al discloses the use of TiCr coating as a hard material layer on a steel substrate forming pro-jections of a chuck for a dental cutting tool. It is not stated how the TiCr alloy is deposited.

In addition the use of a TiCr coating on a sintered aluminium alloy (Al- 12%Si-2%Fe-1%Zr-0.5%Mg-3%A1N by weight), more specifically on inner diameter surfaces of an inner rotor of a gear rotor set mounted on an oval drive shaft is disclosed in EP-A-0907023. The method of applying the coating which is intended to avoid wear and damage occurring is not dis-closed.

Finally WO 03/049086 describes the use of a 3Onm thick TiCr layer as a non-magnetic interlayer in a recording medium having a substrate, a soft magnetic interlayer, the TiCr non-magnetic interlayer and a hard magnetic recording layer. The purpose of the TiCr layer is to prevent magnetic inter-action between the soft magnetic layer and the hard magnetic recording layer. The substrate can be a suitable glass, ceramic, glass-ceramic poly-meric material or a composite or laminate of these materials. The soft magnetic layer can comprise a 50 to 400nm thick layer of Co, CoZr, CoZrCr, CoZrNb, CoFe, Fe, FeN, FeSiA1, FeS1A1N FeCoC etc. The hard magnetic layer is typically a lOnm to 25nm thick layer of a Co based alloy including one or more of the elements selected from the group consisting of Cr, Fe, Ta, Ni, Mo, Pt, V, Nb, Ge, B and Pd, iron oxides or a (C0X/Pd or Pt) multilayer structure where n is an integer from about 10 to 25 each of the alternating thin layers of Co-based magnetic alloy is about 0.2 to 0.35nm thick, X is an element from the group consisting of Cr, Ta, B, Mo, Pt, W and Fe and each of the alternating Pd or Pt layers is about 0.lnm thick. A DLC layer (diamond like carbon) can also be provided as a protec-tive overcoat layer, i.e. providing wear resistance.

The object of the present invention is to propose a new use of such a bi-nary coating.

In accordance with the present invention there is provided a novel use of a binary coating including a layer of at least first and second metallic ele- ments in the form of Ti and Cr as a top layer or an underlayer for corro-sion protection on a metallic substrate consisting of any one of iron, steel, aluminium or an aluminium alloy, magnesium or a magnesium alloy or brass or copper or Zamak or any die-cast material.

It has surprisingly been found that a TiCr underlayer, when used with a preferred top layer system of a wear resistant material having a columnar structure can improve the mechanical properties since it breaks the co-lumnar structure of the top layer system and gives a desirable smoother top surface (less cauliflower like) which in turn yields better wear resis- tance against mechanical wear, especially against abrasive wear. Fur-thermore it has been found that the TiCr binary coating, with or without a top layer or top layer system, has excellent properties as a corrosion pro-tective layer. The reason seems to be that there are no voids where water or other liquids can leak through to initiate corrosion in the layer or layer system or corrosion of the substrate material.

The coating used preferably comprises either a layer of TiCr with a sub-stantially constant composition or a graded TiCR layer, e.g. a base layer (adhesion layer) of Cr and a layer of graded composition consisting of Cr and Ti with the proportion of Ti in the layer increasing from the interface with the base layer to a proportion of Ti greater than that of Cr at the boundary of the graded layer remote from the base layer.

Although the use of a graded TiCr layer is possible it is by no means es-sential. A binary layer with between typically 45 and 70% Cr, especially between 52 and 62% Cr, has been found to be satisfactory, i.e. a layer having a homogenous composition of TiCr rather than a graded composi-tion.

When used as an underlayer the boundary of the graded layer forms an interface to one or more further layers forming a top coating or top coating system respectively. The one or more further layers can for example corn-prise at least one layer of one or more of the following materials: carbon, DLC, nitrides, carbonitrides and carbides of Zr, Ti, Cr, Al or mixtures thereof as well as any known decorative or colored coatings.

The coating serves to protect the substrate from at least one of oxidation, sulfidation, acid corrosion and alkali corrosion, including saline corrosion.

Not only Ti can be used as the first metallic component and Cr as the sec-ond metallic component but rather the first metallic component can be selected from the group comprising Al, Ti, Cr, Si and W whereas the sec-ond component can be selected from the group comprising Cr, Ni; Mo, Nb and B. Al] the pairs of metallic binary coatings recited above are electri-cally conductive.

The binary coating used for corrosion protection is present in at least one of amorphous, nanocrystalline or mixed nanocrystalline form. When used for corrosion protection it can have a thickness in the range from 5Onm to l0jim. As used herein the term "nanocrystalline" signifies crystals which are randomly orientated and have a size under 1 Onm, preferably in the size range between 2nm and lOnm, especially between 3nm and lOnm.

More specifically, when used with a base layer of Cr the thickness of the Cr base layer lies in the range from lOnm to 100 nm and the thickness of the graded layer lies in the range from 4Onm to about 10pm.

The binary coating of the invention is preferably deposited by a PVD proc-ess, for example a PVD process such as a magnetron sputtering process, a HIPIMS process, another ion assisted sputtering process and a plasma assisted vapor deposition process, such as a plasma assisted CVD process (PACVD) or by a combined PVD and PACVD process.

In addition the deposition process for the top layer or some or all of the layers of a top layer system are selected from the group comprising PVD processes, CVD processes, plasma assisted PVD processes, plasma as-sisted CVD processes, electron beam vapor deposition processes, galvanic processes, powder coating processes, dying processes, chemical processes and painting processes.

Thus the use of the binary coating of the invention as an underlayer is not restricted to PVD or CVD processes where similar processes are used to deposit the top layer or top layer system but instead the underlayer can basically be used with all kinds of top layers or top layer systems. Thus a metal substrate coated with the underlayer of the present invention can also be provided with a powder coating or plastic coating, e.g. as a decora-tive coating to color the sheet metal part. Alternatively it can be dyed to color the part, for example to give it a particular sheen or can be painted using any of the known painting processes to provide a decorative finish.

Chemical processes can also be used, for example etching processes in order to improve the bond of the top layer or to chemically modify the top surface of the binary layer to obtain a specific color or desired function.

The coating and optionally any top layer or top layer system can also be subjected to a thermal treatment such as an annealing operation or to a chemical treatment such as an oxidation treatment.

The invention can be used for corrosion protection of a material used in a corrosive industrial environment such as in a chemical reaction apparatus or in a galvanic apparatus or on an electrically conductive element in a battery or fuel cell. It can also be used in corrosive environments such as underwater, especially when exposed to salt water, or with food processing machinery for acidic or alkaline food components or environments.

The invention and a possible method of applying a binary coating in ac-cordance with the invention will now be explained in further detail by way of example and with reference to specific embodiments as illustrated in the accompanying drawings in which are shown: Fig. 1 a schematic cross-section through a substrate provided with a binary coating in accordance with the present teaching, Fig. 2 a schematic cross-section similar to that of Fig. 1 but used as an underlayer provided with a top layer, Fig. 3 a schematic cross-section similar to that of Fig. 1 but provided with a top layer system and Fig. 4 a metastable phase diagram for the equilibrium between the liquid, bcc and amorphous phase of a graded TiCr layer.

Turning now to Fig. 1 there can be seen a substrate 10, for example in the form of a sheet metal part of steel, which is coated on one surface with a binary coating 12 of TiCr in accordance with the present invention. It is possible to provide such coating on both sides of the sheet metal part and indeed on all sides of the sheet metal part if corrosion protection is re-quired on all sides. Moreover, the part need not be a sheet metal part but could be any metallic component which needs to be protected against cor-rosion. In this example the binary coating 12 more specifically comprises a base layer 14 of chromium which is used to provide good adhesion be-tween the layer 16 of TiCr forming the upper part of the layer 12 and the steel substrate. The proportion of Cr in the TiCr layer 16, which could be a graded TiCr layer but in this case is a homogenous TiCr layer is between and 75% and preferably between 52 and 62%.

If it is a graded layer then the percentage of Cr preferably reduces from About 100% at the interface between layers 14 and 16 to about 52 to 62% at the free surface of the layer 16. Fig. 2 shows a diagram similar to Fig. I but in this case a further layer 18 of, in this example, TiN is applied on top of the corrosion protection layer 12 in order to give the article a gold- coloured appearance, TiN being frequently used for this purpose in addi-tion to providing a hard surface which improves the abrasion resistance of the article concerned. If desired a thin layer of real gold (not shown) can be deposited on top of the layer 18. Such a gold-plated article can be benefi-cial because if the relatively soft real gold layer wears away locally, the colour-matched TiN layer beneath it disguises the fact that the gold top layer has partly worn away.

Fig. 3 basically shows the same article as Fig. 2 provided with the binary coating 12 in accordance with the present invention, but in this case it has a top layer system 20, 22 provided for decorative purposes. More spe-cifically, the layer 20 could, for example, be a TiN layer and the layer 22 could be a gold layer.

As is usual, when manufacturing the layer system 12 of Fig. 1 by a PVD process, the substrate would be cleaned prior to depositing the Cr layer.

This can take place by a usual etching process in a magnetron sputtering plant or, if the coating apparatus used is a so-called HIPIMS apparatus, then cleaning of the substrate can also be carried out in a HIPIMS clean-ing mode which is again well known per se.

Analysis of the deposited coatings 14 and 16 have shown the following when the coating 16 is realized as a coating of TiCr of graded composition: It appears from HR-TEM analysis that the initial Cr base or adhesion layer 14 is crystalline, having a bcc A2 crystallographic structure. When intro-duced as an alloying element into a Ti alloy it acts as a p-stabilizer and this allows the metastable formation of the f3-Ti bcc phase, which is re-tamed at room temperature providing the Cr concentration is higher than 7.4 at%. In the compositional range of a Cr-Ti solid solution one expects a bcc A2 substitutional solid solution (3-phase), a metastable B2 phase and a C15 Laves phase within the compositional range. Nevertheless, it has been observed that Ti-Cr solid solutions can undergo inverse melting (solid state amorphization, SSA) after high temperature annealing of me-chanically alloyed samples and sputtered films. For the Ti-Cr system used here it appears that the metastable phase diagram shown in Fig. 4 ap-plies. This indicates that, when varying the composition from a Cr-rich to a Ti-rich Ti-Cr solid solution, phase transitions are expected to occur as follows: Cr-rich 13- p amorphous + 13 -amorphous -amorphous + 13 -Ti-rich 13, where the pure amorphous phase is expected at around 48 at.% Ti con-centration. A progressive variation of the lattice constant seems to occur within the A2 crystal in areas which are progressively rich in Ti and finally the interlayer loses its crystallinity at a Ti concentration at around 60 at.%. It may be that these changes in composition are also in some way responsible for the dense Ti-Cr structure having no voids which ensures that, for example, water or other liquids and gases cannot readily reach the substrate and lead to corrosion there.

Claims

What we claim is: 1. Use of a binary coating including a layer of at least first and second different metallic elements in the form of Ti and Cr as a top layer or an underlayer for corrosion protection on a metallic substrate con-sisting of any one of iron, steel, aluminium or an aluminium alloy, magnesium or a magnesium alloy or brass or copper or Zamak or any die-cast material.
2. Use in accordance with claim 1 wherein the coating comprises either a layer of TiCr with a substantially constant composition or a graded TiCR layer, e.g. a base layer (adhesion layer) of Cr and a layer of graded composition consisting of Cr and Ti with the proportion of Ti in the layer increasing from the interface with the base layer to a proportion of Ti greater than that of Cr at the boundary of the graded layer remote from the base layer.
3. Use in accordance with claim 2 wherein the proportion of Ti at the said boundary comprises essentially 90 to 100% excluding impuri-ties.
4. Use in accordance with claim 2 or claim 3 wherein the boundary of the graded layer comprises a free surface of the coated substrate.
5. Use in accordance with claim 1 or claim 2 wherein the boundary of the graded layer forms an interface to one or more further layers forming a top coating or top coating system respectively.
6. Use in accordance with claim 5 wherein said one or more further layers comprises at least one layer of one or more of the following materials: carbon, DLC, nitrides, carbonitrides and carbides of Zr, Ti, Cr, Al or mixtures thereof as well as any known decorative or colored coatings.
7. Use in accordance with any one of the preceding claims wherein the binary coating serves to protect the substrate from at least one of oxidation, sulfidation, acid corrosion and alkali corrosion, including saline corrosion.
8. Use in accordance with any one of the preceding claims wherein the binary coating is present in at least one of amorphous, nanocrystal-line or mixed amorphous and nanocrystalline form.
9. Use in accordance with any one of the preceding claims wherein the binary coating has a thickness in the range from 50 nm to 10 pm.
10. Use in accordance with claim 2 wherein the thickness of the Cr base layer lies in the range from 10 nm to 100 nm and the thickness of the graded layer lies in the range from 40 nm to about 10 pm.
11. Use in accordance with any one of the preceding claims of a coating deposited by a PVD process, or by a plasma assisted CVD process (PACVD) or by a combined PVD and PACVD process.
12. Use in accordance with claim 11, wherein the PVD process is one of magnetron sputtering, a HIPIMS process, another ion assisted sput-tering process and a plasma assisted vapor deposition process.
13. Use in accordance with claim 5, wherein the deposition process for the top layer or some or all of the layers of a top layer system are se-lected from the group comprising PVD processes, CVD processes, plasma assisted PVD processes, plasma assisted CVD processes, electron beam vapor deposition processes, galvanic processes, pow-der coating processes, dying processes, chemical processes and painting processes.
14. Use in accordance with any one of the preceding claims wherein the coating and optionally any top layer or top layer system has been subjected to a thermal treatment such as an annealing operation or to a chemical treatment such as an oxidation treatment.
15. Use in accordance with any one of the preceding claims for corro-sion protection of a material used in a corrosive industrial environ- ment such as in a chemical reaction apparatus or in a galvanic ap-paratus or on an electrically conductive element in a battery or fuel cell.
16. Use substantially as herein described with reference to and as illus-trated in the accompanying drawings.