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On the properties of physically vapour-deposited Ti-Al-V-N coatings
Thin Solid Films, 153 (1987) 83-90
ON THE Ti-Al-V-N 0.
PROPERTIES COATINGS*
83
OF PHYSICALLY
VAPOUR-DEPOSITED
KNOTEK
Lehrstuhl B und Institut fir Werksto#kunde, Rheinisch- Westfalische Technische Hochschule Aachen, Templergraben 55. D-5100 Aachen (F.R.G.) T. LEYENDECKER
Cemecoat G.m.6.H.. Jiilicher Strape 336, D-5100 Aachen (F.R.G.) F. JUNGBLUT
Lehrsluhl B und Inslitut fir Werkstoflunde, Templergraben 55, D-5100 Aachen (F.R.G.)
Rheinisch- Westfalische Technische Hochschule Aachen,
(Received March 25,1987) Wear-resistant coatings of the quaternary system Ti-Al-V-N exhibit superior wear properties when deposited onto cemented carbides. Previous investigations of multicomponent hard coatings have demonstrated their advantages compared with chemically vapour deposited films currently used on cutting tools. A number of wear tests on both carbide and high speed steel tools indicated that, for example, (Ti, A1)N and (Ti, Zr)N coatings possess superior wear resistance. The resistance of quaternary Ti-Al-V-N coatings is even higher than that of ternary hard compound films. Since coatings of the Ti-Al-V-N system crystallize in an Etc. TiN structure, the alloying agents aluminium and vanadium have a significant influence on their physical and chemical properties. Microstructural, X-ray diffraction, microprobe and micrographic analyses confirm the importance of the aluminium and vanadium content of the multicompound hard coating. In addition, wear tests with coated tools show the effect of the alloying agents on the wear resistance of the coating.
1.
INTRODUCTION
The rapid development of innovative high performance hard material thin films is mainly determined by the advantageous properties of multicomponent hard material systems. One path to the further development of conventional hard material thin films lies in depositing compounds with complex structures, i.e. ternary or quaternary mixed phases. The ability to vary the composition, structure and state of equilibrium of compounds over a wide range through the use of magnetron cathode sputtering as a physical vapour deposition (PVD) process permits direct comparison of the constitution, structure and wear resistance of hard material coatings. * Paper presented at the 14th International U.S.A., March 23-27, 1987.
Conference on Metallurgical Coatings, San Diego, CA,
0 Elsevier Sequoia/Printed in The Netherlands
0.
84
KNOTEK,
T. LEYJZNDECKER,
F. JUNGBLUT
Using the known hard material TIN as a base, alteration of the hard material composition by the incorporation of further elements in the form of either metals (zirconium, aluminium) or metalloids (carbon, oxygen) leads to further improvements in wear resistancelp5. This is impressively documented by wear tests on coated tools. Equivalent or superior wear behaviour has been demonstrated for films prepared by PVD compared with those prepared by chemical vapour deposition (CVD)3,6-8. Owing to its atomic radius, titanium forms solid solutions with numerous other elements. According to Hume-Rotheryg, elements whose atomic radii differ by less than 15% will form mixed crystals by substitution, whereas those with differences in atomic radius of more than 40% compared with the base element will form them by inclusion. Apart from the atomic radius, the chemical bonding (electronegativity) of the atoms and their position in the periodic table will influence the solubility and phase equilibria lo . A further important requirement for the formation of substitutional mixed crystals is that the lattices should be of comparable types, so that phase data for the relevant Ti-Al-V-N systems need to be known”. Unfortunately, no phase diagrams are available for the quaternary system, so diagrams for the corresponding ternary or binary systems have to be used. A study of the incompletely researched ternary systems Ti-Al-N and Ti-V-N provides only partial references to phases deposited by cathode sputtering”. In previous investigations, metastable f.c.c. (Ti, Al)N phases with excellent wear resistance properties have been deposited. Kiinig et al.” discovered a stable (Ti, V)N phase with superior wear resistance in the Ti-V-N system. These good results, and other considerations, indicated that extension of the Ti-Al-N ternary system to include vanadium as a fourth alloying agent would be a promising line of research. The addition of even a small amount of vanadium produced quaternary Ti-Al-V-N films with still further improved wear resistance characteristicsi3. The present contribution reports on innovative hard materials in the Ti-Al-V-N system. The influence of aluminium and vanadium is of particular interest. An account of developments in the composition of these quaternary hard material compounds precedes a report on the wear tests themselves. 2.
EXPERIMENTAL
PROCEDURE
The multicomponent Ti-Al-V-N hard coatings were deposited using a Leybold-Heraeus Z 400 S magnetron cathode sputtering installation working in the d.c. reaction mode with nitrogen as the reactive atmosphere. A stable plasma was reliably initiated by applying a voltage to the target in an argon atmosphere. The sputtering power was set for a target load of 10 W cmm2. The composition of the compounds was easily varied by altering the composition of the target and such process parameters as the reactive atmosphere partial pressure, the substrate temperature and the substrate bias voltage. A simple and reliable method was used to vary target composition. Bolts made of the alloying agent material were inserted in the anticipated sputter zone of the target. Electron
PROPERTIES OF PVD Ti-Al-V-N
COATINGS
85
beam microanalysis of the deposited film provides more exact data on the composition of this mechanical target. A ternary diagram clearly shows the compositions of the targets used to produce the quaternary Ti-Al-V-N films (Fig. 1). The concentrations of the reactively deposited films sputtered with these targets fall into the vanadium-rich Ti-Al-V-N and aluminium-rich Ti-Al-V-N ranges.
Fig. 1. Target compositions.
Sintered hard metals of IS0 group Ml5 (82 wt.% WC, 11 wt.% (Ti,Ta,Nb)C, 7 wt.% Co) were used as the substrate material. Micrographic tests provided initial fundamental data on film thickness, hardness and adhesion. These were supplemented by X-ray diffraction to determine the coating structure and phase composition of the deposited Ti-Al-V-N films. Quantitative electron probe microanalysis (EPMA) furnished data on coating composition. Information on the light element content (nitrogen and oxygen) was particularly useful in permitting reliable interpretation of the properties of the Ti-Al-V-N material under investigationi4. Annealing tests at 1000 “C in a vacuum furnace (less than lop4 mbar) provided data on the stability of the compounds. Subsequently, wear testing (tool life turning tests) was carried out by Krupp Widia G.m.b.H. (Essen), and indicates the performance capability of Ti-Al-V-N coatings on hard metal disposable inserts. 3. RESULTS An extremely stable sputtering process was achieved, irrespective of the target composition employed. Aluminium-rich Ti-Al-V-N films were deposited at target loads of 10 W cm-’ and coating rates of 8 urn h-‘. Rates as high as 9 urn h-i were achieved for vanadium-rich Ti-Al-V-N films. In general, film thicknesses were between 6 and 7 urn. Aluminium-rich Ti-Al-V-N films showed the typical violet colour of (Ti, Al)N, whereas vanadium-rich Ti-Al-V-N showed a yellow-gold colouring, similar to that of TIN. Adhesion tests (scratch test method) revealed critical loads above 75 N for all
86
0. KNOTEK,
T. LEYENDECKER,
F. JUNGBLUT
Ti-Al-V-N coatings on hard metal. No notable differences in adhesion between coatings of varying composition were observable in the high hardness range of interest for wear protection. Figure 2 illustrates the dependence of the microhardness on the nitrogen partial pressure for aluminium-rich Ti-Al-V-N films. Peak hardnesses of 3000 HV 0.05 were measured for these coatings, If aluminium-rich Ti-Al-V-N films are deposited substoichiometrically, there is a clear drop in hardness of the resulting high aluminium content coatings to 1500 HV 0.05. At an aluminium content of 12 at.% the hardness does not fall below 2000 HV 0.05. The dependence of the hardness on the nitrogen partial pressure for vanadiumrich Ti-Al-V-N films is presented in Fig. 3. The flat curves with no pronounced hardness maximum are notable, and may indicate uncritical stoichiometry behaviour. If the vanadum content is increased, in this instance from 4 to 12 at.%, the hardness decreases from 3000 to 2600 HV 0.05.
1201.%V
/ 2
! ndrogen
parllal
, 6 pressure/
1 8 argon
parlid
1
I
2000
I
I
I
IO % pressure
12
0
nllrogen
5 parl,al
IO press”re/aiga”
IS parl1ol
46 Dressure
20
Fig. 2. Microhardness of nitrogen to argon.
of aluminium-rich
Ti-AI-V-N
coatings
as a function
of the partial
pressure
ratio
Fig. 3. Microhardness of nitrogen to argon.
of vanadium-rich
TikAI-V-N
coatings
as a function
of the partial
pressure
ratio
X-ray structural analysis of the quaternary Ti-Al-V-N coatings indicates an f.c.c. lattice structure of the TiN. Under the influence of the aluminium and vanadium alloying agents, however, the X-ray spectrum is displaced towards higher angles, with the Ti-Al-V-N cell much reduced in size compared with that of the TiN. Apparently, aluminium and vanadium atoms are partly substituted for titanium atoms in the TiN lattice, leading to the formation of a (Ti,Al,V)N mixed phase. Since the atomic radii of both aluminium and vanadium are smaller than that of titanium, the cubic cell is reduced in size when these alloying elements are incorporated. The greater the amounts of aluminium and vanadium incorporated in the cell, the smaller is the lattice parameter. This confirms results of previous investigations into (Ti,Al)N coating structures, where titanium atoms of the TiN lattice were likewise partly replaced by aluminium atoms, and the (Ti,Al)N cells exhibited smaller lattice parameters14. The difference between the lattice parameter of the Ti-Al-V-N cell and that of
PROPERTIES
OF
PVD
Ti-Al-V-N
87
COATINGS
the TiN cell is plotted in Fig. 4 as a function of the content of aluminium and vanadium alloying agent. There is an approximately linear reduction in the size of the Ti-Al-V-N cell, indicated as the lattice parameter difference AE (Aa = &.-d T,AIVN)rwith increasing alloy content. As may be seen from Fig. 4, a reduction in cell size for aluminium-rich Ti-Al-V-N is possible only up to a ratio of alloying element to titanium (measured in atomic per cent) of 1.1. The lattice parameter difference is then about 0.008 nm.
r
0.010
I
'0
I
02
I
I
I
I
OL 06 0.8 10 01-% mebll~c oddhans /at-% 11
Fig. 4. Lattice parameter
I
I
I
2
difference as a function
I.1
of the ratio of aluminium
plus vanadium
to titanium.
If this saturation limit of the Ti-Al-V-N cell is exceeded by a further increase in aluminium content, no further reduction in size of this cubic cell is observable. At these high aluminium contents of 30 at.%, X-ray amorphous AlN appears to be present with the Ti-Al-V-N cell, though X-ray analysis revealed no second phase alongside the cubic (Ti, Al, V)N. If stability is tested by subjecting both vanadium-rich and aluminium-rich Ti-Al-V-N coatings to annealing for 3 h in a vacuum furnace (less than 10e4 mbar) at lOOO”C, the metastable character of aluminium-rich hard material films is manifested. In the Ti-Al-V-N films of high aluminium content in particular, X-ray analysis after annealing showed a second phase which could be attributed to AlN. The lattice parameter also increases to a certain extent, indicating aluminium diffusion. The aluminium oxide phases shown in previous investigations of annealed (Ti, Al)N coatings could not be demonstrated in this case. This result is attributed to the low oxygen contents of the Ti-Al-V-N coatings (1 at.% 0). A distinctly more stable thermal behaviour is exhibited by vanadium-rich Ti-Al-V-N coatings. No other phases, apart from the (Ti,Al,V)N mixed phase determined previously, were observed after annealing. Other results of electron probe microanalysis confirm a (Ti,Al,V)N mixedphase compound for aluminium-rich and vanadium-rich Ti-Al-V-N coatings, which give good performances in terms of wear resistance (microhardness, scratch test, tool life) when they have a slightly substoichiometric composition with metal contents between 52 and 57 at.%. This stability range is determined by the aluminium or vanadium content. The marked reduction in size of the f.c.c. cell impedes interstitial inclusion of the metalloid, necessarily shifting the stability towards higher metal contents.
0.
88 4.
WEAR
KNOTEK,
T. LEYJZNDECKER,
F. JUNGBLUT
TESTS
The behaviour of high performance hard material coatings in tool life turning tests is of great interest. Figure 5 shows the performance of hard metal disposable inserts coated with vanadium-rich Ti-Al-V-N. Crater wear is significantly below that for the CVD Tic-TiN specimen used for comparison. These good results are strengthened by the comparison between the coating thickness of the CVD specimen (12 urn) and the considerably lower thickness of all Ti-Al-V-N coatings (6-7 urn). Ti-Al-V-N films with a high vanadium content displayed particularly good stability and resistance to diffusion, and thus low crater wear. Flank wear limits the extent to which the vanadium content of Ti-Al-V-N films can be increased (Fig. 6). Despite the unfavourable position of the flank in relation to the target (only the first face and target were parallel to one another during deposition), it is again the Ti-Al-V-N films with a somewhat lower vanadium content which exhibit a high resistance to wear. 03,
Fig. 5. Crater
cut; cutting C 60 N).
depth of hard metals with vanadium-rich Ti-AI-V-N data, aP = 15 mm, f = 0.28 mm; U, = 224 m min-I;
Fig. 6. Flank wear of hard metals with vanadium-rich
Ti-Al-V-N
1
I
coatings (operation, turning, clean tool, coated insert M 15; material, coatings
(details as for Fig. 5).
Aluminium-rich Ti-AI-V-N coatings show their superiority in tool life turning tests at lower aluminium contents (Fig. 7). Here too, there is only a small amount of first-face crater wear on the coated hard metals compared with that on the CVD specimen, despite the significantly lower coating thickness (6-7 urn compared with 12 urn). An excess aluminium content (greater than 30 at.%) leads to partially reduced crater wear resistance, despite good results in micrographic investigations. This result is attributed to the fact that the solubility limit for aluminium and vanadium in the TIN lattice may be ,attained or exceeded (Fig. 5). The metastable character of Ti-Al-V-N films of high aluminium content, already encountered in annealing tests at temperatures around 1000 “C, is again manifested here. All aluminium-rich Ti-Al-V-N films also possess high resistance to abrasion, as shown by the flank wear (Fig. 8). The wear resistance of CVD Tic-TiN is clearly exceeded.
PROPERTIES
OF PVD
cuillng
Ti-Al-V-N
89
COATINGS
culling llme
INme
Fig. 7. Crater depth of hard metals with aluminium-rich Fig. 8. Flank wear of hard metals with aluminium-rich
Ti-AI-V-N Ti-AI-V-N
coatings coatings
(details as for Fig. 5). (details as for Fig. 5).
5. DISCUSSION
Interesting observations result from an attempt to combine the superior properties of vanadium-rich Ti-AI-V-N coatings with the advantages of their aluminium-rich equivalents. A high vanadium content evidently improves the stability and resistance to diffusion of the coating, as emphasized by the excellent crater wear results. Vanadium inclusions restrict the drop in hardness values, but encourage brittleness of the material, as shown by the low resistance of the tool flank. Aluminium-rich Ti-Al-V-N coatings maintain their resistance to flank wear even at high hardness values. Their crater wear resistance, however, is slightly poorer. If the target compositions employed (Fig. 1) are included in thecomparison, it becomes apparent that the coatings with the best performances lie in the range between the aluminium-rich and vanadium-rich Ti-Al-V-N films. 6. SUMMARIZING
REMARKS
Previous investigations have shown that the addition of small amounts of vanadium increases the wear resistance of (Ti, Al)N coatings. We examined the effect of both aluminium and vanadium on the coating properties of quaternary Ti-Al-V-N films, Machining tests on Ti-Al-V-N-coated hard metals confirmed the differing influences of vanadium and aluminium. It was shown that the addition of vanadium increases the wear resistance on the first face, but tends to increase the brittleness of the material. In contrast, coatings with added aluminium possess good wear resistances, even on the tool flank. This emphasizes the ductility of the coating. It is evident that further improvements are still possible in the direction of maximum wear resistance. REFERENCES
1
0. Knotek,
H. Reimann
and W. Bosch, Metall., 37 (1983) 233.
90
2 3 4 .5 6 I 8
9 10 11 12 13 14
0. KNOTEK,
T. LEYENDECKER,
F. JUNGBLUT
W. D. Miinz, D. Hofmann and K. Hartig, Thin SolidFilms, 96 (1982) 79. W. D. Miinz and D. Hofmann, MetalloberJliiche, 37 (1983) 279. 0. Knotek and W. Bosch, Observations on Ti(C,N) coating by reactive sputtering, Met. Powder Rep., 39 (1984) 406. 0. Knotek, W. Bosch and T. Leyendecker, Tribologie, Vol. 9, Springer, Berlin, 1985. W. D. Miinz, CEI Course Nitride and Carbide Coatings, Leyboid-Heraeus Special Reprint ll-SO 7.2, September, 1985. (Laboratoire Suisse de Recherches Horlogeres, Neuchatel, Switzerland). W. D. Miinz, J. Vat. Sci. Technol. A, 4 (6) (1986) 2717. 0. Knotek, W. D. Miinz and T. Leyendecker, Industrial deposition of binary and ternary nitrides and carbonitrides of titanium, zirconium and aluminium, 6th Inr. Conf. Solid Surfaces, Baltimore, MD, October 8,1986, in J. Vat. Sci. Technol. A, in the press. W. Hume-Rothery and G. V. Raynor, The Structure of Metals and Alloys, Institute of Metals, London, 1969. J. Kornilov, in R. I. Jaffee and N. E. Promise1 (eds.), The Science, Technology and Application of Titanium, Pergamon, Oxford, 1966, p. 407. H. Holleck, Bin&e und terndre Carbid- und Nitridsysteme der Hbergangsmetalle, Gebr. Borntrager, Berlin, 1984. U. Koniget al., Untersuchungsber. UB2016, 1984, (Krupp Widia GmbH, Postfach 102161, D-4300 Essen 1). 0. Knotek, M. Biihmer and T. Leyendecker, J. Vat. Sci. Technol. A, 4 (6) (1986) 2695. 0. Knotek, W.-G. Burchard, P. Karduck and T. Leyendecker, Beitr. elektronenmikroskop. Direktabb. Oberji., 19(1986) 211.
ON THE Ti-Al-V-N 0.
PROPERTIES COATINGS*
83
OF PHYSICALLY
VAPOUR-DEPOSITED
KNOTEK
Lehrstuhl B und Institut fir Werksto#kunde, Rheinisch- Westfalische Technische Hochschule Aachen, Templergraben 55. D-5100 Aachen (F.R.G.) T. LEYENDECKER
Cemecoat G.m.6.H.. Jiilicher Strape 336, D-5100 Aachen (F.R.G.) F. JUNGBLUT
Lehrsluhl B und Inslitut fir Werkstoflunde, Templergraben 55, D-5100 Aachen (F.R.G.)
Rheinisch- Westfalische Technische Hochschule Aachen,
(Received March 25,1987) Wear-resistant coatings of the quaternary system Ti-Al-V-N exhibit superior wear properties when deposited onto cemented carbides. Previous investigations of multicomponent hard coatings have demonstrated their advantages compared with chemically vapour deposited films currently used on cutting tools. A number of wear tests on both carbide and high speed steel tools indicated that, for example, (Ti, A1)N and (Ti, Zr)N coatings possess superior wear resistance. The resistance of quaternary Ti-Al-V-N coatings is even higher than that of ternary hard compound films. Since coatings of the Ti-Al-V-N system crystallize in an Etc. TiN structure, the alloying agents aluminium and vanadium have a significant influence on their physical and chemical properties. Microstructural, X-ray diffraction, microprobe and micrographic analyses confirm the importance of the aluminium and vanadium content of the multicompound hard coating. In addition, wear tests with coated tools show the effect of the alloying agents on the wear resistance of the coating.
1.
INTRODUCTION
The rapid development of innovative high performance hard material thin films is mainly determined by the advantageous properties of multicomponent hard material systems. One path to the further development of conventional hard material thin films lies in depositing compounds with complex structures, i.e. ternary or quaternary mixed phases. The ability to vary the composition, structure and state of equilibrium of compounds over a wide range through the use of magnetron cathode sputtering as a physical vapour deposition (PVD) process permits direct comparison of the constitution, structure and wear resistance of hard material coatings. * Paper presented at the 14th International U.S.A., March 23-27, 1987.
Conference on Metallurgical Coatings, San Diego, CA,
0 Elsevier Sequoia/Printed in The Netherlands
0.
84
KNOTEK,
T. LEYJZNDECKER,
F. JUNGBLUT
Using the known hard material TIN as a base, alteration of the hard material composition by the incorporation of further elements in the form of either metals (zirconium, aluminium) or metalloids (carbon, oxygen) leads to further improvements in wear resistancelp5. This is impressively documented by wear tests on coated tools. Equivalent or superior wear behaviour has been demonstrated for films prepared by PVD compared with those prepared by chemical vapour deposition (CVD)3,6-8. Owing to its atomic radius, titanium forms solid solutions with numerous other elements. According to Hume-Rotheryg, elements whose atomic radii differ by less than 15% will form mixed crystals by substitution, whereas those with differences in atomic radius of more than 40% compared with the base element will form them by inclusion. Apart from the atomic radius, the chemical bonding (electronegativity) of the atoms and their position in the periodic table will influence the solubility and phase equilibria lo . A further important requirement for the formation of substitutional mixed crystals is that the lattices should be of comparable types, so that phase data for the relevant Ti-Al-V-N systems need to be known”. Unfortunately, no phase diagrams are available for the quaternary system, so diagrams for the corresponding ternary or binary systems have to be used. A study of the incompletely researched ternary systems Ti-Al-N and Ti-V-N provides only partial references to phases deposited by cathode sputtering”. In previous investigations, metastable f.c.c. (Ti, Al)N phases with excellent wear resistance properties have been deposited. Kiinig et al.” discovered a stable (Ti, V)N phase with superior wear resistance in the Ti-V-N system. These good results, and other considerations, indicated that extension of the Ti-Al-N ternary system to include vanadium as a fourth alloying agent would be a promising line of research. The addition of even a small amount of vanadium produced quaternary Ti-Al-V-N films with still further improved wear resistance characteristicsi3. The present contribution reports on innovative hard materials in the Ti-Al-V-N system. The influence of aluminium and vanadium is of particular interest. An account of developments in the composition of these quaternary hard material compounds precedes a report on the wear tests themselves. 2.
EXPERIMENTAL
PROCEDURE
The multicomponent Ti-Al-V-N hard coatings were deposited using a Leybold-Heraeus Z 400 S magnetron cathode sputtering installation working in the d.c. reaction mode with nitrogen as the reactive atmosphere. A stable plasma was reliably initiated by applying a voltage to the target in an argon atmosphere. The sputtering power was set for a target load of 10 W cmm2. The composition of the compounds was easily varied by altering the composition of the target and such process parameters as the reactive atmosphere partial pressure, the substrate temperature and the substrate bias voltage. A simple and reliable method was used to vary target composition. Bolts made of the alloying agent material were inserted in the anticipated sputter zone of the target. Electron
PROPERTIES OF PVD Ti-Al-V-N
COATINGS
85
beam microanalysis of the deposited film provides more exact data on the composition of this mechanical target. A ternary diagram clearly shows the compositions of the targets used to produce the quaternary Ti-Al-V-N films (Fig. 1). The concentrations of the reactively deposited films sputtered with these targets fall into the vanadium-rich Ti-Al-V-N and aluminium-rich Ti-Al-V-N ranges.
Fig. 1. Target compositions.
Sintered hard metals of IS0 group Ml5 (82 wt.% WC, 11 wt.% (Ti,Ta,Nb)C, 7 wt.% Co) were used as the substrate material. Micrographic tests provided initial fundamental data on film thickness, hardness and adhesion. These were supplemented by X-ray diffraction to determine the coating structure and phase composition of the deposited Ti-Al-V-N films. Quantitative electron probe microanalysis (EPMA) furnished data on coating composition. Information on the light element content (nitrogen and oxygen) was particularly useful in permitting reliable interpretation of the properties of the Ti-Al-V-N material under investigationi4. Annealing tests at 1000 “C in a vacuum furnace (less than lop4 mbar) provided data on the stability of the compounds. Subsequently, wear testing (tool life turning tests) was carried out by Krupp Widia G.m.b.H. (Essen), and indicates the performance capability of Ti-Al-V-N coatings on hard metal disposable inserts. 3. RESULTS An extremely stable sputtering process was achieved, irrespective of the target composition employed. Aluminium-rich Ti-Al-V-N films were deposited at target loads of 10 W cm-’ and coating rates of 8 urn h-‘. Rates as high as 9 urn h-i were achieved for vanadium-rich Ti-Al-V-N films. In general, film thicknesses were between 6 and 7 urn. Aluminium-rich Ti-Al-V-N films showed the typical violet colour of (Ti, Al)N, whereas vanadium-rich Ti-Al-V-N showed a yellow-gold colouring, similar to that of TIN. Adhesion tests (scratch test method) revealed critical loads above 75 N for all
86
0. KNOTEK,
T. LEYENDECKER,
F. JUNGBLUT
Ti-Al-V-N coatings on hard metal. No notable differences in adhesion between coatings of varying composition were observable in the high hardness range of interest for wear protection. Figure 2 illustrates the dependence of the microhardness on the nitrogen partial pressure for aluminium-rich Ti-Al-V-N films. Peak hardnesses of 3000 HV 0.05 were measured for these coatings, If aluminium-rich Ti-Al-V-N films are deposited substoichiometrically, there is a clear drop in hardness of the resulting high aluminium content coatings to 1500 HV 0.05. At an aluminium content of 12 at.% the hardness does not fall below 2000 HV 0.05. The dependence of the hardness on the nitrogen partial pressure for vanadiumrich Ti-Al-V-N films is presented in Fig. 3. The flat curves with no pronounced hardness maximum are notable, and may indicate uncritical stoichiometry behaviour. If the vanadum content is increased, in this instance from 4 to 12 at.%, the hardness decreases from 3000 to 2600 HV 0.05.
1201.%V
/ 2
! ndrogen
parllal
, 6 pressure/
1 8 argon
parlid
1
I
2000
I
I
I
IO % pressure
12
0
nllrogen
5 parl,al
IO press”re/aiga”
IS parl1ol
46 Dressure
20
Fig. 2. Microhardness of nitrogen to argon.
of aluminium-rich
Ti-AI-V-N
coatings
as a function
of the partial
pressure
ratio
Fig. 3. Microhardness of nitrogen to argon.
of vanadium-rich
TikAI-V-N
coatings
as a function
of the partial
pressure
ratio
X-ray structural analysis of the quaternary Ti-Al-V-N coatings indicates an f.c.c. lattice structure of the TiN. Under the influence of the aluminium and vanadium alloying agents, however, the X-ray spectrum is displaced towards higher angles, with the Ti-Al-V-N cell much reduced in size compared with that of the TiN. Apparently, aluminium and vanadium atoms are partly substituted for titanium atoms in the TiN lattice, leading to the formation of a (Ti,Al,V)N mixed phase. Since the atomic radii of both aluminium and vanadium are smaller than that of titanium, the cubic cell is reduced in size when these alloying elements are incorporated. The greater the amounts of aluminium and vanadium incorporated in the cell, the smaller is the lattice parameter. This confirms results of previous investigations into (Ti,Al)N coating structures, where titanium atoms of the TiN lattice were likewise partly replaced by aluminium atoms, and the (Ti,Al)N cells exhibited smaller lattice parameters14. The difference between the lattice parameter of the Ti-Al-V-N cell and that of
PROPERTIES
OF
PVD
Ti-Al-V-N
87
COATINGS
the TiN cell is plotted in Fig. 4 as a function of the content of aluminium and vanadium alloying agent. There is an approximately linear reduction in the size of the Ti-Al-V-N cell, indicated as the lattice parameter difference AE (Aa = &.-d T,AIVN)rwith increasing alloy content. As may be seen from Fig. 4, a reduction in cell size for aluminium-rich Ti-Al-V-N is possible only up to a ratio of alloying element to titanium (measured in atomic per cent) of 1.1. The lattice parameter difference is then about 0.008 nm.
r
0.010
I
'0
I
02
I
I
I
I
OL 06 0.8 10 01-% mebll~c oddhans /at-% 11
Fig. 4. Lattice parameter
I
I
I
2
difference as a function
I.1
of the ratio of aluminium
plus vanadium
to titanium.
If this saturation limit of the Ti-Al-V-N cell is exceeded by a further increase in aluminium content, no further reduction in size of this cubic cell is observable. At these high aluminium contents of 30 at.%, X-ray amorphous AlN appears to be present with the Ti-Al-V-N cell, though X-ray analysis revealed no second phase alongside the cubic (Ti, Al, V)N. If stability is tested by subjecting both vanadium-rich and aluminium-rich Ti-Al-V-N coatings to annealing for 3 h in a vacuum furnace (less than 10e4 mbar) at lOOO”C, the metastable character of aluminium-rich hard material films is manifested. In the Ti-Al-V-N films of high aluminium content in particular, X-ray analysis after annealing showed a second phase which could be attributed to AlN. The lattice parameter also increases to a certain extent, indicating aluminium diffusion. The aluminium oxide phases shown in previous investigations of annealed (Ti, Al)N coatings could not be demonstrated in this case. This result is attributed to the low oxygen contents of the Ti-Al-V-N coatings (1 at.% 0). A distinctly more stable thermal behaviour is exhibited by vanadium-rich Ti-Al-V-N coatings. No other phases, apart from the (Ti,Al,V)N mixed phase determined previously, were observed after annealing. Other results of electron probe microanalysis confirm a (Ti,Al,V)N mixedphase compound for aluminium-rich and vanadium-rich Ti-Al-V-N coatings, which give good performances in terms of wear resistance (microhardness, scratch test, tool life) when they have a slightly substoichiometric composition with metal contents between 52 and 57 at.%. This stability range is determined by the aluminium or vanadium content. The marked reduction in size of the f.c.c. cell impedes interstitial inclusion of the metalloid, necessarily shifting the stability towards higher metal contents.
0.
88 4.
WEAR
KNOTEK,
T. LEYJZNDECKER,
F. JUNGBLUT
TESTS
The behaviour of high performance hard material coatings in tool life turning tests is of great interest. Figure 5 shows the performance of hard metal disposable inserts coated with vanadium-rich Ti-Al-V-N. Crater wear is significantly below that for the CVD Tic-TiN specimen used for comparison. These good results are strengthened by the comparison between the coating thickness of the CVD specimen (12 urn) and the considerably lower thickness of all Ti-Al-V-N coatings (6-7 urn). Ti-Al-V-N films with a high vanadium content displayed particularly good stability and resistance to diffusion, and thus low crater wear. Flank wear limits the extent to which the vanadium content of Ti-Al-V-N films can be increased (Fig. 6). Despite the unfavourable position of the flank in relation to the target (only the first face and target were parallel to one another during deposition), it is again the Ti-Al-V-N films with a somewhat lower vanadium content which exhibit a high resistance to wear. 03,
Fig. 5. Crater
cut; cutting C 60 N).
depth of hard metals with vanadium-rich Ti-AI-V-N data, aP = 15 mm, f = 0.28 mm; U, = 224 m min-I;
Fig. 6. Flank wear of hard metals with vanadium-rich
Ti-Al-V-N
1
I
coatings (operation, turning, clean tool, coated insert M 15; material, coatings
(details as for Fig. 5).
Aluminium-rich Ti-AI-V-N coatings show their superiority in tool life turning tests at lower aluminium contents (Fig. 7). Here too, there is only a small amount of first-face crater wear on the coated hard metals compared with that on the CVD specimen, despite the significantly lower coating thickness (6-7 urn compared with 12 urn). An excess aluminium content (greater than 30 at.%) leads to partially reduced crater wear resistance, despite good results in micrographic investigations. This result is attributed to the fact that the solubility limit for aluminium and vanadium in the TIN lattice may be ,attained or exceeded (Fig. 5). The metastable character of Ti-Al-V-N films of high aluminium content, already encountered in annealing tests at temperatures around 1000 “C, is again manifested here. All aluminium-rich Ti-Al-V-N films also possess high resistance to abrasion, as shown by the flank wear (Fig. 8). The wear resistance of CVD Tic-TiN is clearly exceeded.
PROPERTIES
OF PVD
cuillng
Ti-Al-V-N
89
COATINGS
culling llme
INme
Fig. 7. Crater depth of hard metals with aluminium-rich Fig. 8. Flank wear of hard metals with aluminium-rich
Ti-AI-V-N Ti-AI-V-N
coatings coatings
(details as for Fig. 5). (details as for Fig. 5).
5. DISCUSSION
Interesting observations result from an attempt to combine the superior properties of vanadium-rich Ti-AI-V-N coatings with the advantages of their aluminium-rich equivalents. A high vanadium content evidently improves the stability and resistance to diffusion of the coating, as emphasized by the excellent crater wear results. Vanadium inclusions restrict the drop in hardness values, but encourage brittleness of the material, as shown by the low resistance of the tool flank. Aluminium-rich Ti-Al-V-N coatings maintain their resistance to flank wear even at high hardness values. Their crater wear resistance, however, is slightly poorer. If the target compositions employed (Fig. 1) are included in thecomparison, it becomes apparent that the coatings with the best performances lie in the range between the aluminium-rich and vanadium-rich Ti-Al-V-N films. 6. SUMMARIZING
REMARKS
Previous investigations have shown that the addition of small amounts of vanadium increases the wear resistance of (Ti, Al)N coatings. We examined the effect of both aluminium and vanadium on the coating properties of quaternary Ti-Al-V-N films, Machining tests on Ti-Al-V-N-coated hard metals confirmed the differing influences of vanadium and aluminium. It was shown that the addition of vanadium increases the wear resistance on the first face, but tends to increase the brittleness of the material. In contrast, coatings with added aluminium possess good wear resistances, even on the tool flank. This emphasizes the ductility of the coating. It is evident that further improvements are still possible in the direction of maximum wear resistance. REFERENCES
1
0. Knotek,
H. Reimann
and W. Bosch, Metall., 37 (1983) 233.
90
2 3 4 .5 6 I 8
9 10 11 12 13 14
0. KNOTEK,
T. LEYENDECKER,
F. JUNGBLUT
W. D. Miinz, D. Hofmann and K. Hartig, Thin SolidFilms, 96 (1982) 79. W. D. Miinz and D. Hofmann, MetalloberJliiche, 37 (1983) 279. 0. Knotek and W. Bosch, Observations on Ti(C,N) coating by reactive sputtering, Met. Powder Rep., 39 (1984) 406. 0. Knotek, W. Bosch and T. Leyendecker, Tribologie, Vol. 9, Springer, Berlin, 1985. W. D. Miinz, CEI Course Nitride and Carbide Coatings, Leyboid-Heraeus Special Reprint ll-SO 7.2, September, 1985. (Laboratoire Suisse de Recherches Horlogeres, Neuchatel, Switzerland). W. D. Miinz, J. Vat. Sci. Technol. A, 4 (6) (1986) 2717. 0. Knotek, W. D. Miinz and T. Leyendecker, Industrial deposition of binary and ternary nitrides and carbonitrides of titanium, zirconium and aluminium, 6th Inr. Conf. Solid Surfaces, Baltimore, MD, October 8,1986, in J. Vat. Sci. Technol. A, in the press. W. Hume-Rothery and G. V. Raynor, The Structure of Metals and Alloys, Institute of Metals, London, 1969. J. Kornilov, in R. I. Jaffee and N. E. Promise1 (eds.), The Science, Technology and Application of Titanium, Pergamon, Oxford, 1966, p. 407. H. Holleck, Bin&e und terndre Carbid- und Nitridsysteme der Hbergangsmetalle, Gebr. Borntrager, Berlin, 1984. U. Koniget al., Untersuchungsber. UB2016, 1984, (Krupp Widia GmbH, Postfach 102161, D-4300 Essen 1). 0. Knotek, M. Biihmer and T. Leyendecker, J. Vat. Sci. Technol. A, 4 (6) (1986) 2695. 0. Knotek, W.-G. Burchard, P. Karduck and T. Leyendecker, Beitr. elektronenmikroskop. Direktabb. Oberji., 19(1986) 211.