CA2328152A1 - A low friction coating for a cutting tool - Google Patents

Abstract

The present invention relates to a composition for use as a coating on a cutting tool, which composition comprises: (i) a first material selected from TiNx, TiA1Nx, TiA1YCrNx and CrNx; and (ii) a second material selected from MoSz and WSz, wherein x is from 0.5 to 1.5, z is from 0.8 to 2.2 and the atomic ratio (y) of Mo or W to Ti or Cr is from 0.1 to 0.8.

Description

WO 99/55930 PCTlGB99/01311 A low friction coating for a cutting tool The present invention relates to a composition for use as a.coating on a cutting tool and, in particular, to a low friction, wear-resistant coating formed from a hard phase of, for example, TiN and a soft, lubricating phase of, for example, MoS2.
Sputtered MoS2 films were developed some three decades ago and have since been used principally for space lubrication applications. Over the last five years these films have been increasingly used as tool coatings for cutting and shaping applications, resulting in significant improvements to tool-life, finish and productivity. Interest is now growing in MoS2 tool coatings for dry machining applications, where little or no lubricants and/or cooling fluids are used. Although dry machining is now industrially feasible using the latest generation of TiAIN
coatings, the additional use of an MoS2 overlayer has been proposed to provide the lubricating effect previously obtained by the use of environmentally-harmful lubricant fluids.
The ever increasing demands placed on these relatively soft films has resulted in considerable effort to improve endurance properties. Deposition parameters were first optimised for pure MoS2 films.
Later, the beneficial effect of metallic additives, such as Ni, Pb and Ti, was applied to sputtered coatings by co-deposition and in the form of multilayers, bringing significant improvements to film density, lubrication and endurance. In addition, the multilayers display better resistance to oxidation, which can degrade lubrication and endurance by depleting superficial sulphur and undermining coating adhesion. However, because the overlayers are still relatively soft, they are often consumed in the critical break-in phase of a cutting tool and lose their effectiveness soon afterwards.
TiB2 has been investigated as a hard matrix material with a lubricating phase of MoS2. Whilst hardness and low friction properties can generally be obtained, such coatings can sometimes fail owing to the brittleness of the TiH2 phase.
The present invention aims to address the problems associated with the prior art and to provide dense, relatively hard and wear-resistant coatings with a sufficiently high enough concentration of MoS2 I5 or WSZ to obtain self lubrication. This is preferably achieved by co-depositing a lubricating material with a hard coating material.
Accordingly, in a first aspect the present invention provides a composition for use as a coating on a cutting tool, which composition comprises:
(i) a first material selected from TiNx, TiAINX, TiAlYCrNx and CrNx, and (ii) a second material selected from MoS= and WSZ, wherein x is from 0.5 to 1.5, z is from 0.8 to 2.2 and the atomic ratio (y) of Mo or W to Ti or Cr is from 0.1 to 0.8.
In a first preferred embodiment, x is from 0.5 to 1.5, z is from 1 to 2.2 and the atomic ratio (y) of Mo or W to Ti or Cr is from 0.2 to 0.8. Advantageously, x is from 0.7 to 1.3, more preferably 0.8 to 1.2, still more preferably equal to or approximately equal to 1 for even higher hardnesses and lower coefficients of friction. For the same reasons, z is preferably from 1.2 to 2, more preferably from 1.4 to 1.8, still mote preferably equal to or approximately equal to 1.6. Similarly, y is preferably from 0.2 to 0.5, more preferably from 0.2 to 0.4, still more preferably equal to or approximately equal to 0.3. The composition according to this embodiment has a hardness typically of at least 14 GPa, more typically from 15 to 23 GPa, still more typically from 16 to 22 GPa.
In a second preferred embodiment, x is from 0.5 to 1.5, z is from 0.8 to 2.2 and the atomic ratio (y) of Mo or W to Ti or Cr is between 0.1 to 0.2 when z is from 0.8 to 2.2, and is from 0.1 to 0.8 when z is between 0.8 and 1. x is advantageously from 0.7 to 1.3, more preferably 0.8 to 1.2, still more preferably equal to or approximately equal to 1 for even higher hardnesses and lower coefficients of friction. For the same reasons z is preferably from 0.8 to 1.4.
Similarly, y is preferably from 0.12 to 0.18, more preferably from 0.14 to 0.16, still more preferably equal to or approximately equal to 0.15. The composition according to this embodiment has a hardness typically of at least 14 GPa, more preferably from 22 to 32 GPa, still more preferably from 25 to 30 GPa.
For all embodiments of the present invention it will be appreciated that the first material may comprise more than one of TiNx, TiAlNx, TiAlYCrNxand CrNx and the second material may comprise one or both of MoSz and WSZ .
The composition according to all of the embodiments of the present invention will generally have a friction coefficient of from 0.08 to 0.3, more generally from 0.08 to 0.2, and a hardness typically of at least 14 GPa.
In a third preferred embodiment of the present invention, the composition comprises:
(i) a first material selected from TiNx, TiAINX, TiAlYCrNx and CrNX, and (ii) a second material selected from MoSZ and WSZ, wherein x is equal to or approximately equal to 1, z is equal to or approximately equal to 1.6, and the atomic ratio (y) of Mo or W to Ti or Cr is from 0.2 to 0.4, and wherein the composition has a friction coefficient of from 0.08 to 0.2, preferably from 0.08 to 0.12, and a hardness of from 16 to 22 GPa, preferably from 18 to 22 GPa, more preferably from 20 to 22 GPa.
In a fourth preferred embodiment of the present invention, the composition comprises:
(i) a first material selected from TiNx, TiAINX, TiAlYCrNx and CrNx, and (ii) a second material selected from MoSZ and WSZ, wherein x is equal to or approximately equal to 1, z is equal to or approximately equal to 1.1, and the atomic ratio (y} of Mo or W to Ti or Cr is between 0.1 and 0.2, and wherein the composition has a friction coefficient of from 0.08 to 0.2, preferably from 0.08 to 0.12, and a hardness of from 22 to 32 GPa, preferably from 25 to 30 GPa, more preferably approximately equal to 27 GPa.

The microstructure of a coating according to the present invention typically comprises a soft, lubricating phase of MoSZ or WSZ in a hard matrix phase of TiNx, TiAlNx, TiAlYCrNx or CrNx, with nanometre grain size, typically of from 1 to 10 nm. The presence of a 'lubricant reservoir' substantially throughout the coating thickness allows lubrication to be maintained even as the coating wears away. An additional advantage arises because the MoSZ or WSZ, phase is incorporated within a hard matrix and this provides a degree of oxidation protection.
In the present invention, TiN, TiAlN, TiAlYCrN or CrN are used as a hard phase because, in addition to their outstanding mechanical properties (superhardness and high toughness), these nitrides display relatively good tribological properties compared with other hard coatings. For example, TiN has a friction coefficient typically quoted between 0.4 and 0.8 against steel, or even less than 0.2 when rubbing against itself or certain hard counter-faces, such as A1203.
The present invention also provides a wear-resistant, self-lubricating coating comprising a composition as herein described and, additionally, an article having such a coating.
The present invention also provides a cutting tool comprising a substrate having one or more coatings thereon, wherein at least one of the said coatings comprises a composition as herein described.
It will be appreciated that the substrate may have more than one such coating, for example a first coating of TiNx and MoSZ and a second coating of, for example, TiAlNx and WSZ. It will alsa be appreciated that compositional variations may exist within each coating. For example, a gradient coating may be applied comprising TiNX and MoS=, wherein the MoSZ
content is zero (or at least approaching zero) at the substrate surface and is continuously increased with distance into the coating (and away from the substrate) until the required level is reached.
The substrate material may be formed from any of the conventional cutting tool materials, such as, for example, a hard metal, a high speed steel or a cermet.
A layer consisting essentially of Ti, TiAl, TiAlYCr, Cr, TiN, TiAlN, TiAlyCrN or CrN is advantageously disposed between a surface of the substrate and the one or more coatings, since this has been found to improve adhesion between the coatings) and the substrate. The choice of the material for this intermediate layer is advantageously based on the nitride constituent of the coating. For example, if the coating comprises TiNX, then the intermediate layer would preferably be formed from Ti or TiN. In a preferred aspect, a first layer is provided consisting essentially of Ti, TiAl, TiAlYCr or Cr and this layer is disposed adjacent a surface of the substrate, and a second layer consisting essentially of a nitride of the material of the first layer, i.e. TiN, TiAlN, TiAlYCrN or CrN, is disposed between the first layer and the one or more coatings as herein described.
Again, the choice of the material for the first layer is advantageously based on the nitride constituent of the coating. For example, if the coating comprises TiAINX, then the first layer would preferably be formed from an alloy of Ti and A1. In this manner a three-component adhesion-promoting underlayer may advantageously be used. For example, a first layer of Ti may be deposited on to the surface of the substrate. Next, a layer of TiN may be deposited on to the first layer. Next, a gradient layer of TiN and MoSz may be deposited, starting off with 0~ MoSZ (or at least approaching zero) and then continuously increasing the MoSz content to that of the functional TiNx(MoSz)y self-lubricating layer as herein described.
Whilst the cutting tool according to the present invention may be used with a lubricant and/or a cooling fluid, it is particularly suited to dry-machining applications, i.e. no lubricant or cooling fluid, or micro-lubrication applications, where very little lubricant and/or cooling fluid is/are required.
The present invention also provides for the use of a composition as herein described as a low-friction, wear-resistant coating. Whilst the coating composition will generally be used in the manufacture of cutting tools, it will be appreciated that it can be used in any area requiring low-friction and good wear-resistance.
In a another aspect of the present invention, there is provided a process for forming a wear resistant, low friction coating on a substrate, which process comprises the steps of:
(i) providing a substrate (ii) placing the substrate in a deposition chamber having means to deposit first and second deposition materials simultaneously on to a surface of the substrate, wherein the first deposition material is selected from TiN, TiAlN, TiAlYCrN and CrN and the second deposition material is selected from MoS2 and WS2;
(iii) co-depositing the first and second deposition materials on to the surface of the substrate under conditions to form a coating having a composition as herein described.
The deposition chamber may comprise a sputtering system, for example an unbalanced do magnetron sputtering system, in which the means to deposit the first and second deposition materials simultaneously preferably includes a target comprising the first deposition material and a target comprising the second deposition material. Alternatively, a composite target may be used. In this way, Tin and MoS2, for example, can be co-sputtered on to the surface of the substrate. The composite target may take the form of a mosaic target having, for example, one or more TiN
portions adjacent one or more MaS2 portions.
Magnetron sputter ion plating is described in detail in GB-2 258 343.
The substrate is typically maintained at a temperature of from 80 to 500°C, preferably from 100 to 300°C, more preferably from 150 to 250°C and still more preferably approximately 200°C during deposition.
Deposition is preferably carried out in a vacuum or an inert gas atmosphere. For example, an argon atmosphere at a pressure of from 0.1 to 1 Pa Ar, preferably from 0.4 to 1 Pa, has been found to be suitable.
Deposition is generally carried out using a power density typically in the range of from 5 to 20 Wcm2, preferably from 5 to 15 Wcm2. Experiments have been conducted using a target current of from 0.5 to 1.5 A
(equivalent to a target voltage of from 250 to 800 V) and preferably approximately 1 A (equivalent to a target voltage of approximately 520 V).
A coating thickness of typically from 0.5 to 5 ~m _ g _ can be produced in less than an hour.
A bias voltage of from 0 to -200 V, preferably from -50 to -150 V, more preferably from -75 to -125 V, is typically applied to the substrate during deposition. A bias voltage of approximately -100 V
has been found to provide the best results.
Alternatively, it may be envisaged to deposit the hard matrix, TiN for example, reactively by introducing a sufficient quantity of nitrogen into the process gas whilst sputtering from a Ti target. It may also be envisaged to reactively deposit the MoSZ, for example, by reactive deposition using a Mo target and the addition of a sulphur containing process gas such as HzS .
Alternatively, an arc/sputter hybrid device may be used to deposit, for example TiN and MoS2 simultaneously on the surface of the substrate. In this manner, the hard phase of, for example, TiN, is deposited from a titanium cathodic arc and MoS2, for example, is simultaneously deposited by magnetron sputtering. In this example, it is necessary to use a mixed nitrogen/argon mixture. The nitrogen allows the reactive deposition of TiN in conjunction with the Ti arc source whilst the argon provides an efficient sputtering gas for the MoS2. In effect, nitrogen is not particularly effective for the sputtering of MoS2.
Furthermore, any other deposition process which would allow the deposition of the first deposition material in conjunction with magnetron sputtering of the second deposition material, such as plasma-assisted CVD or photon-induced deposition, may be used.

When a cathodic arc device is used to deposit the hard phase, for example TiN, in conjunction with a magnetron sputtering device to deposit the lubricating material, for example MoS2, the arc current should be adjusted so as to obtain a suitable TiN deposition rate with respect to the MoS2 deposition rate in order to produce a coating within the given composition.
Typically, the arc current is at least 50 A. In this embodiment of the process, the pressure inside the deposition chamber is provided by the nitrogen and argon process gases. A pressure of from 0.1 to 1 Pa is preferred with the volumic flow ratio of nitrogen to argon preferably being approximately 10:1, so as to provide a suitable compromise between the reactive deposition of TiN with nitrogen and the sputtering of MoS2 using argon. It will be appreciated by those skilled in the art that whilst magnetron sputter deposition is currently the most common method for depositing MoSz, as arc technology improves, it may also be possible to deposit the MoS2 using an MoS2 arc source. Alternatively, it may also be possible to deposit MoS2 reactively using a Mo arc source and a sulphur containing process gas, such as HzS.
The present invention also provides a process for machining, for example milling, a cast iron component, which process comprises the step of machining a component using a cutting tool as herein described.
The step of machining may be carried out in the absence of a lubricant or a cooling fluid (dry machining) or, alternatively, using a minimum amount of a lubricant and/or a cooling fluid (micro-lubrication).
It will be appreciated that machining may be applied to a variety of materials as well as cast iron components, for example carbon steels, stainless steels, aluminium, copper, and including alloys thereof.
The presence of a lubricant reservoir substantially throughout the coating thickness allows lubrication to be maintained even as the coating wears away.
The present invention will now be described further, by way of example, with reference to the following drawings, in which:
Figure 1 is a schematic representation of a co-deposition process according to the present invention;
Figure 2 shows the variation of atomic ratios for the overall composition TiNx(MoSZ)Y with substrate position and substrate bias as measured by GDOES (x and y), EDX (y and z) and XPS (x and z).
Figures 3(a) and (b) shows SEM cross-sections for coatings deposited on Mo substrates in positions -30 at 0 (a) and -100 V (b) .
Figures 4(a) and (b) shows the variation in X-ray diffraction spectra with substrate position for 0 V
bias (a) and -100 V bias (b). Each spectrum is labelled with substrate position in mm.
Figure 5 shows the variation in hardness and friction coefficient as a function of substrate position for 0 V and -100 V bias. Friction values are taken over the whole sliding distance, except for biased coatings produced in positions -40 to -65, for which the first 50 m of unstable friction are excluded from the average calculation.

Figure 6 are friction curves showing a typical behaviour for biased coatings in positions -40 to -65, characterised by an initial maximum in friction (curves a, b and c), and friction behaviour typical of all other coatings, characterised by practically featureless low friction (curve d).
Figure 7 is a schematic top view of a deposition chamber according to the present invention;
Figure 8 shows room temperature pin-on-disk test results for TiN-MoSZ coatings as compared to TiN
standard coatings using either 6 mm diameter 100Cr6 steel or A1203 balls as counterfaces.
Figure 9 shows comparative lifetime test results for uncoated HSS drills, TiN-coated drills and TiN-MoS= coated drills.
Film deposition may be performed using an unbalanced do magnetron sputtering system 1, schematically represented in Figure 1 and described in detail by P. Losbichler and C. Mitterer in Surf. Coat.
Technol., 97 (1997), 568-574. A composite TiN-MoS2 target 5 was used, made up of two halves cut from 150 mm diameter TiN and MoS2 targets, 6 and 7 respectively, which were bonded to a water-cooled backing plate 8.
Substrates 10 were mechanically polished and degreased stainless steel rectangles (10 x 20 mm). The substrates 10 were fixed to a holder 11, located 6 cm above the target 5 and placed at various positions between -75 and + 75 mm (this number refers to the horizontal distance between the centre of the substrate 10 and the target's 5 central dividing line, negative on the TiN side and positive on the MoS2 side). The chamber 2 was pumped down to a base pressure of 10-' Pa with a turbo-molecular pump 3.

Prior to film deposition, the target 5 was sputter cleaned for 5 minutes behind a shutter 13 and substrates 10 were sputter etched for 20 minutes with an Ar pressure of 3.5 Pa and a do voltage of -1500 V.
Deposition was then carried out, with the substrate 10 temperature maintained at 200°C by a resistance heater 12, using 0.7 Pa Ar pressure, 1 A target 5 current (around 520 V) and either 0 or -100 V substrate 10 bias. Deposition time was 45 minutes and resulted in a coating thickness in the 0.5 to 3 ~cm range, depending on substrate 10 position.
The overall coating chemistry and the phase composition was investigated by XPS on a Cameca-Nanoscan 50, incorporating a MAC2 semi-imaging analyser, set at an energy resolution of 0.5 eV. The unmonochromated MgKa source was operated at 12 kV and 30 mA. A rastered 3 keV, 0.2 /.c,A Ara ion beam was employed to remove the surface oxide, the 0 is peak being monitored until it no longer decreased. The energy scale was calibrated using the Cu 2p3,2 and Au 4f"2 peaks at 932.67 and 83.98 eV respectively.
Quantification of the spectra was performed using relative sensitivity factors determined from Ti Mo, TiN and MoS2 standards. Due to the preferential sputtering of S, the S content was estimated from the Mo 3d5,2 (sulphide) peak position in the spectra from the native surface.
Coating composition was also determined using Glow Discharge Optical Emission Spectroscopy (GDOES), calibrated using TiN and alloyed Mo-Ti standards, and EDX analysis, X-ray microstructural analysis of the samples was performed by Glancing Angle X-ray Diffraction (GAXRD) using an unmonochromated copper source at an incident angle of 1.0°, a high precision beam collimation and sample positioning system, and a solid state detector to maximise the signal/noise ratio.
Film morphology was studied using a Cambridge Instruments Stereoscan 360 scanning electron microscope (SEM).
Hardness and Young's modulus were determined by an ultra-low load depth-sensing nanoindenter (Nanoindenter II, from Nano Instruments Inc.) described in detail by R. Gilmore, M.A. Baker; P.N.
Ginbson and W. Gissler in Surf. Coat, Technol. (in press). A pin-on-disk tribometer (CSEM) was used to determine the friction coefficient. Counterface material was 6 mm diameter steel ball and track radius was 3.5 mm. A normal load of 1 N and a sliding speed of 0.05 m/s were used over a sliding distance of 250 m in laboratory air and at an ambient temperature (24°C
tl) .
While not wishing to be constrained by theory, the following comments are made in arr attempt to throw some light on the results.
Coating composition as a function of substrate position and bias voltage is shown in Figure 2.
Because coatings from position +20 to +75 tended to rapidly spall off once removed from the vacuum chamber, characterisation results are available for substrate positions -75 to +15 only. The MoSZ content decreases progressively from a maximum value of approximately 66~ at position +15 to a lower Limit of around 18~ at position -75. The MoSZ content is calculated as 100 x 1/(1 + y) for the overall composition TiNx(MoSZ)Y. The TiN is generally stoichiometric with only very slight variation with substrate position and no measurable effect due to substrate bias. The MoS2 phase is slightly sub-stoichiometric at about MoSl,s, showing relatively little variation with substrate position or bias.
SEM cross sections of selected coatings (corresponding to max hardness) are shown in Figures 3(a) and (b) for 0 V and -100 V bias. The films appear to be, dense with good adhesion and a fine grained structure.
Figure 4 shows GAXRD spectra for the various coatings with superimposed MoSZ and TiNx phases to separate. Where easily observed, the TiN phase is obviously nanocrystalline, and it shaws a more ordered structure under the TiN half of the target (from positions -15 to -75) with an average crystallite size of the order of several nm. There is a significant shift of the TiN peaks to lower angles for all samples, representing an increase in lattice parameter. This might be attributable to the incorporation of Mo and/or S in the cubic structure, though the observed shift is more probably caused by N
being substituted by S, since it is known that Mo substitution does not produce a large change in the lattice parameter. There appears to be a rapid widening of the peak width from position -45 to position -25. This is assumed to be due to a rapid decrease of the TiN grain size owing to competition with the formation of extended MoSZ zones which becomes more likely the nearer the substrate is to the MoS2 half of the target. A diffraction spectrum typical of sputter-deposited MoS2 is evident at position -15. The structure of the MoS2 which gives rise to such a spectrum has been described as consisting of randomly stacked 001 planes with fluctuations of the distance in the c direction between these planes. There is very little extension of the structure in the a-b direction. As such, this represents neither a proper nanocrystalline nor a real amorphous structure. It is likely that in such a structure a considerable concentration of Ti and N
atoms could be accommodated, and this may further account for the sudden reduction in the TiN
crystallite size which is observed in the GAXRD
spectrum when the MoS2 structure starts to form, since Ti or N atoms arriving at MoS2 growth zones will easily become trapped there. For coatings of low MoSx content, there is a lack of any obvious MoSz 002 peak.
The shape and XPS binding energies of the Ti 2p3,2, N ls, S 2p3,2 and Mo 3d5,2 peaks for the TiN and MoS2 standards (respectively 455.3, 397.4, 162.1 and 229.2 eV) are in good agreement with the corresponding average peak positions for all of the co-sputtered samples (respectively 455.3 ~ 0.1, 397.3 ~ 0.1, 162.1 ~ 0.1 and 228.9 ~ 0.15 eV), indicating in general the presence of separate TiNx and MoSZ phases, also indicated by the GAXRD results. The lower XPS Mo peak position reflects the sub-stoichiometry of the MoS2 phase. The XPS spectra do not allow confirmation of the suspected substitution of Mo and/or S into the TiN
lattice suggested by the GAXRD results possibly because of peak overlapping for S in the form of MoS2 and substituted in the TiN lattice. As a reference for the S 2p3,2 peak position, TiS has a quoted binding energy of 163.5 eV, but has an h.c.p. structure.
Possibly a better reference is f.c.c. MnS (Mn having an electronegativity very similar to Ti) for which the binding energy is quoted as 162.0 eV, which would lead to a peak overlapping that of MoS2. Even for the lowest MoSZ contents (18~), it seems reasonable to conclude that an MoSz phase exists since only a small part could substitute into the TiN lattice. At first sight, this observation appears to be in contradiction with the corresponding GAXRD spectra which shows a lack of any obvious 002 peak at position -45 and positions further from the MoS2 half, despite the high scattering power of Mo. One possible explanation of this would be a complete loss of correlation in the c-direction of the MoSZ structure and this may mean that the MoSZ, which is not incorporated in the TiN
structure, is reduced to single layers, presumably located between the TiN grains.
ZO
Figure 5 shows the hardness and friction results.
Hardness passes through a maximum around position -40, for both the biased and non-biased coatings with maximum average values of about 20 and 17 GPa respectively. The increase in hardness between position +15 and -90 can be attributed to the improving crystal~structure of the TiN phase as the MoSz content diminishes. The systematically higher hardness values in the case of -100 V bias for positions left of 20 may be attributable to the optimum deposition conditions for the TiN phase. It is, however, not clear why the hardness then diminishes for positions to the left of -40. It would rather be expected that hardness continues to increase or stabilise as MoSZ content decreases. It does not appear to be an artefact of the nanoindentation technique which may start to measure substrate hardness as coating thickness falls below 10~ of penetration depth: the results obtained with 200 nm penetration depth were cross-checked with 50 nm results and were in good agreement. It may be that the fall in hardness is principally related to some microstructural change due to changing deposition parameters close to the target's edge related to some shadowing at the target's extremity.
Friction remained low for all studied compositions. Low friction was obtained despite MoSZ
under-stoichiometry and an apparently random basal plane orientation, though this is not in contradiction with the literature, where it is reported that stoichiometries above approximately 1.2 are usually lubricating, and MoS2 is able to preferentially re-orient under the action of friction with basal planes parallel to sliding direction. The average friction coefficient was generally close to 0.1 and showed no direct correlation with hardness. Friction curves were typically flat (Figure 6(d)) with the notable exception of the biased coatings deposited in positions -40 to -65. For these coatings, the friction coefficient passed through a large and distinctive initial maximum as high as 1.1 (Figures 6(a-c)). The friction coefficient then fell to around 0.2 after about 50 m and remained relatively stable for the remaining 200 m of the test. For samples displaying this atypical behaviour, the friction values shown in Figure 5 are averages taken over the final 200 m to exclude the initial instabilities. The distinctive shape of these curves were reproducible and thought perhaps to be of some significance.
Initial friction maxima are often associated with transfer film formation and the resemblance to friction curves reported for TiN rubbing against steel under fretting conditions is significant. It may be that the observed friction maximum corresponds to the formation of a third body from the TiN coating which is transformed under the action of friction to a sub-stoichiometric form of rutile, resulting in the observed low final friction. It may be that for our biased samples produced in positions -40 to -65, which have the best formed TiN lattice and highest hardness, friction is governed by the TiN phase in the initial stages until the tribo-assisted formation of a lubricious oxide transfer layer allows the friction coefficient to return to a relatively low value.
However, this behaviour apparently disappears at even lower MoS2 contents, possibly due to the fact that the hardness drops away.
The comments above are provided in an attempt to explain the results and are not intended to constrain the present invention by theory.
In another example, film deposition was performed using the system schematically represented in Figure 7. The deposition facility 20 was composed of two opposing arc evaporation Ti sources 25 and two opposing MoS2 magnetron targets 30, which could be operated independently. The samples (drills and flat substances) were positioned in a carousel-type sample holder 35 with three rotational axes. The drills were of the type HSS DIN 338 of 6 mm diameter and 100 mm length and the flat samples were hard metal discs of diameter 23 mm. The flat samples were later used for mechanical and structural characterisation (such as glancing angle X-ray diffractometry, EDX, nanoindentation, pin-on-disk testing, scratch testing) whilst the drills were used for field tests.
Samples were cleaned in an ultrasonic bath in a benzene/alcohol solution. Before deposition, the chamber was evacuated to a residual pressure of 5x10-5 mbar and then heated to 250°C for 30 minutes using infra red heaters 40, 41 and 42. Coating was then performed as follows.
i) An ion etching process was performed in a substantially pure Ar atmosphere at a pressure of approximately 0.01 mbar.
ii) To enhance adhesion, a ~ 5 nm Ti layer was first deposited, also in a pure 0.01 mbar Ar atmosphere, followed by a ~ 100 nm TiN coating deposited reactively in a substantially pure nitrogen atmosphere of approximately 0.001 mbar using approximately 70 A on the Ti arc source.
iii) Then the MoS2 sputter source was operated in an Ar/NZ atmosphere using flow rates of approximately 50 and 500 standard cubic centimetres per minute respectively to maintain the pressure at approximately 0.01 mbar. The power to the MoS2 sputter target was progressively increased from approximately 300 to 1500 Watts whilst decreasing the Ti arc current to approximately 55 A, to create a 300 nm gradient layer of increasing MoS., content.
iv) Finally a coating of about three microns was produced by maintaining the Ti arc current and sputter power at approximately 5.5 A and about 1500 Watts respectively. The bias voltage on the carousel was held at approximately -100 volts.
For comparison, pure TiN coatings were also prepared using steps i) and ii) above, whereby TiN
deposition was continued so as to produce a 3 micron coating.
The chemical composition of the co-deposited coatings was determined using EDX analysis and found to be (TiN) o.e, (MoSZ) 0.13 wherein z is in the range of 0.8 < z < 1.4.
The coating was found to display a f.c.c.
structure typical for TiN with a slightly enlarged lattice parameter as determined by Glancing Angle X-Ray Diffraction.
The hardness of the coatings was determined to be about 27 GPa with a nanoindenter, approximately the same as that measured for the pure TiN coatings. The nanoindenter was calibrated with a Si 111 wafer, assuming for this material a hardness of 11 GPa.
The adhesion was determined with a scratch tester. A critical load value of 105 N was found.
The friction coefficient was measured with a pin-on-disk tester using a steel (100Cr6) or A1203 sphere of diameter 6 mm as a counterface at a sliding speed of 0.1 m/s (track radius 8 mm) and a load of 5 N at room temperature and with a relative humidity of about 40~. Figure 8 displays the friction coefficient of a TiN and a TiN-MoS2 coating as function of the distance. After a short running-in phase the friction coefficient of the TiN-MoSz coating assumes a constant value of less than 0.2 for both types of counterface in comparison to 0.8 for the TiN coating.
All drills have been subjected to a field test under dry machining conditions on a CNC milling machine of type Deckel P2A by drilling holes under the following conditions:
feed rate = 0.32 mm/rev, speed = 1600 rpm, hole depth = 27 mm, work piece = carbon steel C35.
The lifetime was determined by the number of holes drilled before the drill broke. In order to facilitate the running-in phase the first five holes were drilled with a depth of 20 mm. The results are shown in Figure 9, where the lifetimes of several TiN-MoS2 coated drills are compared with uncoated drills and TiN-coated drills. The expected increase in lifetime is observed for the TiN coated drills with respect to the uncoated drills by a factor of approximately four. A further increase in lifetime by about 25~ was observed for the TiN-MoSz coated drills.
The present invention provides a process for producing dense, well adhering coatings which can combine a hardness exceeding about 20 GPa with a friction coefficient of less than about 0.2.
Optimum hardness/friction properties were obtained for films deposited using approximately -100 V substrate bias.
The TiN-MoS2 system has potential for producing well-adhering films comparable with standard TiN tool coatings by the use of a TiN underlayer then graded interlayer.

Claims (21)

CLAIMS:

1. A composition for use as a coating on a cutting tool, which composition comprises;
(i) a first material selected from TiN x, TiAlN x, TiAlYCrN x and CrN x, and (ii) a second material selected from MoS ~ and WS z, wherein x is from 0.5 to 1.5, z is from 0.8 to 1.4 and the atomic ratio (y) of Mo or W to Ti or Cr is from 0.1 to 0.2, and wherein the composition has a hardness of from 22 to 32 GPa and a fiction coefficient of from 0.08 to 0.3.

2. A composition as claimed in claim 1, wherein y is from 0.12 to 0.18, preferably from 0.14 to 0.16, more preferably equal to or approximately equal to 0.15.

3. A composition as claimed in claim 1 or claim 2 having a hardness of from 25 to 30 GPa.

4. A composition far use as a coating on a cutting tool, which composition comprises:
(i) a first material selected from TiN x, TiAlN x, TiAlYCrN x and CrN x, and (ii) a second material selected from MoS z and WS z, wherein x is equal to or approximately equal to 1, z is equal to or approximately equal to 1.6, and the atomic ratio (y) of Mo or W to Ti or Cr is from 0.2 to 0.4, and wherein the composition has a friction coefficient of from 0.08 to 0.2 and a hardness of from 16 to 22 GPa.

5. A composition for use as a coating on a cutting tool, which composition comprises:
(i) a first material selected from TiN x, TiAlN x, TiAlYCrN x and CrN x, and (ii) a second material selected from MoS z and WS z, wherein x is equal to or approximately equal to 1, z is equal to or approximately equal to 1.1, and the atomic ratio (y) of Mo or W to Ti yr Cr is from 0.1 to 0.2, and wherein the composition has a friction coefficient of from 0.08 to 0.2 and a hardness of from 22 to 32 GPa.

6. A composition as claimed in any one of the preceding claims having a microstructure comprising a soft phase of MoS z or WS z in a hard matrix phase of TiN x, TiAlN x, TiAlYCrN x or CrN x.

7. A composition as claimed in any one of the preceding claims, wherein the first material comprises TiN x and the second material comprises MoS z.

8. A wear-resistant, self-lubricating coating comprising a composition as defined in any one of claims 1 to 7.

9. An article having a wear-resistant, self-lubricating coating as defined in claim 8.

10. A cutting tool comprising a substrate having one or more coatings thereon, wherein at least one of the said coatings comprises a composition as defined in any one of claims 1 to 7.

11. A cutting tool as claimed in claim 10, wherein a layer consisting essentially of Ti, TiAl, TiAlYCr, Cr, TiN, TiAlN, TiAlYCrN or CrN is disposed between a surface of the substrate and the one ar more coatings.

12. A cutting tool as claimed in claim 10 or claim 11, wherein a first layer consisting essentially of Ti, TiAl, TiAlYCr or Cr is disposed adjacent a surface of the substrate, and a second layer consisting essentially of a nitride of the material of the first layer is disposed between the first layer and the one or more coatings.

13. A cutting tool as claimed in any one of Claims 10 to 12 which is a micro-lubrication or dry machining cutting tool.

14. Use of a composition as claimed in any one of claims 1 to 7 as a low-friction, wear-resistant coating.

15. A process for forming a wear resistant, low friction coating on a substrate, which process comprises the steps of:
(i) providing a substrate;
(ii) placing the substrate in a deposition chamber having means to deposit first arid second deposition materials simultaneously on to a surface of the substrate, wherein the first deposition material is selected from TiN, TiAlN, TiAlYCrN and CrN and the second deposition material is selected from MoS2 and WS2; and (iii) co-depositing the first and second deposition materials on to the surface of the substrate under conditions to form a coating having a composition as defined in any one of claims 1 to 7.

16. A process as claimed in claim 15, wherein the deposition chamber comprises a sputtering system and the means to deposit first and second deposition materials includes a target comprising the first deposition material and a target comprising the second deposition material or a composite target comprising the first and second deposition materials.

17. A process as claimed in claim 16, wherein the deposition chamber comprises an unbalanced dc magnetron sputtering system.

18. A process as claimed in any one of Claims 15 to 17, wherein the deposition chamber comprises a combined arc evaporation and sputtering system and the means to deposit the first deposition material comprises a suitable metal arc source and a suitable gas and the means to deposit the second deposition material comprises a target comprising the second deposition material.

19. A process as claimed in any one of claims 15 to 18, wherein a bias voltage of from 0 to -200 v is applied to the substrate during deposition, preferably from -54 to -150 V, more preferably approximately -100 v.

20. A process for machining a cast iron component, which process comprises the step of machining a component using a cutting tool as claimed in any one of claims 10 to 12.

21. A process as claimed in claim 20, wherein the step of machining is carried out in the absence of a lubricant or a cooling fluid or using a minimum amount of a lubricant and/or a cooling fluid.