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An investigation of the tribological potential of TiN, CrN and TiN + CrN physical vapor deposited coatings in machine element applications
45
Wear, 170 (1993) 45-53
investigation .of the tribological potential of TiN, CrN and TiN+ CrN physical vapor deposited coatings in machine element applications
An
Y.L. Su and J.S. Lin Department of Mechanical Engineering, National Cheng Kung lJniversi& Tainan 70101 (Taiwan) (Received
February
22, 1993; accepted
June 24, 1993)
Abstract This study investigates, through wear testing, the potential for carburized and tufftrided surface treated steels and the TiN, CrN and TiN+ CrN physical vapor deposited (PVD) surface treatments to be used in the screw and rollers of the variable lead screw transmission mechanism (VLSTM). Indentation test results reveal that the thicker the PVD CrN coating, the lower is its adhesive strength, with the desired coating thickness being 10 pm. In general, all sliding pairs possess the best wear resistance under cutting fluid lubrication. Surface treated steels (upper moving specimen) paired with PVD coated parts possess very poor wear resistance under base oil lubrication, even worse than under dry wear. Suitable sliding pairs for VLSTM applications include: surface treated steels paired with either SKH51 or PVD coated specimens under cutting fluid lubrication; PVD coated specimens paired with SKH51 specimens under cutting fluid lubrication; and PVD coated specimens paired with PVD coated specimens under base oil and cutting fluid lubrication.
1. Introduction The VLSTM [l] is the heart of “shuttleless” textile machinery, and possesses four rollers driven by a fourbar linkage mechanism. The rollers slide against the screw, with the resulting angular accelerating motion of the latter being used as output (Figs. 1 and 2). The problem associated with this mechanism arises from contact loading and the sliding velocity inducing wear in both the screw and rollers. Previous work in this laboratory [2,3] addressed the high wear rate problem by considering ceramic coated and surface treated steels for the rollers and surface treated steels for the screws. PVD nitride and carbide coatings are being increasingly employed in both tribological and corrosion re-
ariable lead screw
Belt whiel
Fig. 1. A schematic
0043-1648/93/$6.00
diagram of the VLSTM.
iable pitch cylindrical
cam
Fig. 2. The variable lead screw is driven by four rollers (3a, 3b, 3c and 3d).
sistant applications [4-6], especially in the metal cutting industry [7-111. The scarcity of work done on TiN, CrN and TiN+ CrN coatings suggests possible applications in machine elements. This study investigates, through wear testing, the potential for the TiN, CrN and TiN + CrN surface treatments to be used in VLSTM screws and rollers. Carburized and tufftrided surface treated steels are also examined. The criterion of compatibility [12] is one of the most important principles in tribological material selection. Suitably mated materials can not only reduce the wear
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of the PVD coated part itself, but also reduce wear in the counterpart. Since TiN and CrN coatings are quite hard, an excessively soft counterpart will easily degrade. Obviously, the choice of a suitable sliding pair warrants careful consideration and further study.
2. Experimental method The compositions of the three materials (JIS standard) used in these experiments are listed in Table 1. The SKII is a high speed steel. The SACM645 contains aluminum, which can form aluminum nitride during nitridation. The SCM415 is a low carbon steel for carburization application. 2.1. Specimen preparation 2.1.1.
TiN coating
Prior to deposition, SKI-I51 (JIS standard) substrates were vacuum heat treated (quenched at 1220 “C, followed by tempering at 560 “C) to HV763 characteristics, and then polished with 0.3 pm A&O, powder to a roughness Rz of less than 1.2 ,um. Titanium nitride PVD coatings were deposited on SISH51 substrates through the use of a multiple cathode arc plasma (CAP) system. Substrate temperatures during deposition were maintained at 400-450 “C. The processing parameters (- 100 V substrate bias voltage, 80 A arc current, 10m2 mbar N, partial pressure, 3,um coating thickness) represent optimal conditions determined previously by the authors [13]. The microhardness (HV25g) of PVD TiN 3pm thick is 2398. 2.1.2. CrN and TiN~3~~ + CrN coatings Chromium nitride PVD coatings were also deposited on SKH51 substrates using a CAP system (Multi-arc Company, USA), Substrate temperatures during deposition were maintained at 400 “C. Processing parameters similar to the above, except that NZ partial pressure was increased to 0.26 mbar, were employed. Optimum coating thickness was determined through dry wear testing of 5 @urn,10 Mm and 15 pm CrN coatings paired with tufftrided specimens.
SACM645 base materials were quenched and tempered to HRC30 characteristics. The tufftrided trcatmcnt is a salt bath nitriding process [ 141 that induces an I-IV1018 surface microhardness to SACM645. A bath temperature of 580 “C and a 3 h treatment time were employed in the tufftriding process. Thermally refined SCM415 (HRC30) was carburized to HRC60 surface hardness. 2.2. Wear testing Wear testing was performed by an SRV oscillating wear machine produced by the Optimol Company, Germany. The coefficient of friction, load, stroke and temperature were recorded in real time by a computerized data acquisition system (DAS). The dimensions of the test specimens and a schematic diagram of the experimental setup are shown in Fig. 3. Note that the upper cylindrical specimen is in contact with the flat side of the lower specimen, and that the former oscillates horizontally in the axial direction. The tests were conducted at room temperature and atmospheric pressure. The relative humidity of the laboratory was approximately 45%-50%. Constant test strokes, normal loads and frequencies of 0.5 mm, 300 N and 50 Hz respectively were employed. Specimens were first run in at 20 N and 30 Hz for 2 min, with the frictional data not being recorded. The total sliding distance under both dry and lubricated wear conditions was approximately 72 m. Base oil and synthetic oil were the lubricants used in this study. The former possesses kinematic viscosities
Oscillating Driving motor Force transducer
2.1.3, S.w$ace treated ~~e&irnen~ Tufftridation and carburization were the surface treatments. Prior to surface treatment, the SCM415 and TABLE
1. Specimen
composition
L
(a)
,1,
recipmcating
(%) (a)front view
Si
MnP
1
S
Al
Ni
Material
C
Cr
MO Fe
SACM645 SCM415 SW51
0.46 0.23 0.5 0.02 0.018 0.8 0.050 1.5 0.2 0.17 0.25 0.69 0.02 0.018 0.037 1.0 0.2 0.85 0.23 0.3 0.02 0.007 0.06 4.1 5.3
Bal. Bal. Bal.
(b)
n upper q Lower
(b)side view
specimen(diameter:lJmm
lengthz22mm)
specimen(diameter:24mm
height:7.9mmf
Fig. 3. (a) A schematic specimen dimensions.
diagram
of the experiment
setup;
(b)
XL. Su, J.S. Lin f Tribdogicai
pote?ztiai of PVD coatings for machine elements
of 138.9 cSt and 14.4 cSt at 40 “C and 100 “C respectively. The latter is a water-based cutting fluid ~ynn’s 983) devoid of sulphur and chlorine. Its specific gravity is 1.018 at 15 “C; the pH value of a 1:20 oil/water solution (diluted for application purposes} is 8.9-9.2. This is typical for medium and heavy cutting applications. Dry wear was also investigated for comparison with lubricated wear performance. 2.3. Related equipment A profilometer was used to measure specimen wear scar dimensions. The amount of wear is evaluated based on wear scar depth for the lower specimen (SKI%1 substrates with and without subsequent CrN or TiN+CrN coatings) and wear scar width for the upper specimen (surface treated steels, TiN and CrN coated specimens). Microhardness testing was used to examine specimen surface and subsurface microhardness. Scanning electron microscopy (SEM) was used for metallographic examination and worn surface observation; X-ray diffraction was employed in identi~ng the preferred orientation of the PVD coatings. Rockwell C scale indentation tests (150 kg), performed on CrN coated specimens of different coating thicknesses, helped to determine the adhesive strength of these PVD coatings 1151.
47
_---.
(a)
0’)
(cl Fig. 4. (a) Microstructure of a tufftrided treated TF-1 specimen; (b) line-scan of Ti distribution for TiN (3 pm) PVD coating; (c) line-scan of Cr distribution for CrN (10 Frn)PVD coating.
3. Results 3.1. Microstructureexamination The microstructure of a tufftride treated specimen is shown in Fig. 4(a), where the secondary electron image (SEI) is on the left and the back scattered electron image (BEI) is on the right. This surface treatment yields a porous r-Fe,N single phase surface zone, consisting of iron, nitrogen and carbon compounds. In addition to good anti-frictional and wear behavior, this compound layer exhibits no tendency to seize. In both electron images, the “tree” structure in the subsurface region of the sample is y’-Fe,N. The surface mo~holo~ of PVD TiN (3 ,um) is shown in Fig. 4(b). Note the presence of many irregularly shaped droplets on the coating surface. The Ti linescan clearly indicates that these particles are titanium rich, which, unfortunately, decreases the coating’s anticorrosion performance and wear resistance under severe wear conditions [16]. Note that the surface morphology of PVD CrN (10 pm) shown in Fig. 4(c) is similar to that of TiN discussed above. The Cr line-scan, in contrast, indicates that the particles are not chromium-rich, being of the same composition as the CrN surface coating. Although no cross-sectional studies of these droplets were made, these particles may well be chromium rich at their core.
(Cl Fig. 5. The indentation cracking of various CrN coating thicknesses: (a) 5 pm; (b) 10 pm; (G) 15 pm.
The results of CrN wear tests revealed that droplets decrease wear resistance, especially during the runningin phase. Note, however, that the anticorrosive behavior of arc-evaporated CrN coatings can still be good [17]. 3.2, Effect of coating thickness Photographs of 5, 10 and 15 pm thick CrN coatings after the Rockwell hardness C scale indentation test (load 150 kg) are shown in Fig. 5. Each of the three CrN coatings possesses radial cracks, and that lateral
cracks becume more visible with an increase in coating thickness. These results indicate that the brittleness of the coating layer increases under high internal stress, while the adhesive strength decreases with an increase in coating thickness. The surface texture of PVD CrN (see Fig. 6) consists only of the (220) phase, while PVD TiN consists of the (111) and (222) phases [13]. These preferred orientations do not alter as the coating thickness is increased. The mi~rohardn~ss of CrN coating with various coating thickness is shown in Fig. 7(a). It seems that as the coating thickness is increased, the microhardness is increased. This is attributed to either the substrate effect at the smaller coating thickness or the increase in internal stresses at greater thickness, as shown in Fig. 5. The relationship between coating thickness and wear depth of CrN coated specimens paired with tufftrided specimens under dry wear conditions is shown in Fig. 7(b). In terms of optimal wear resistance, the CrN coating thickness should be approximately 10 pm. If the coating thickness is 15 pm, although it possesses good wear resistance, the brittleness of the coating layer is increased (as described above). If the coating is too thin (5 pm), it will be easily worn through. 3.3. Wear and fictkm The wear testing results of carburized, tufftrided, TiN (3 pm) and CrN (10 pm) upper specimens paired with SKJSl, TiN (3 pm)/CrN (10 pm) and CrN (10 pm) lower specimens under both dry and lubricated conditions are given in Table 2, In order to ease the inte~r~tation of these results, wear depths of the SKH51, TiN + CrN and CrN lower specimens are plotted --I
t
CrN(ZZ0)
Fig. 6. X-ray diffraction patterns of SIG351 samples with and without subsequent TiN or CrN coatings of different thicknesses: (a) SKEEI; (b) CrN, 5 ,um; (c) CrN, 10 ,um; fd) CrN, 15 pm.
Substrate
IOum
Sum
15um
Coatinp. thickness
(a)
5
10 FiLM
15
TWiCKNESS(um)
Fig. 7. {a) The relation of coating thickness and microhardness of CrN coated samples with various thicknesses (HV25g); (b) the relation of coating thickness and wear depth for CrN coated samples (upper specimens) paired with tufftrided samples (lower specimens) under dry wear conditions.
against the wear scar width of their various upper specimen pairs in Fig. 8, In Fig. S(a), where SKIS1 is the lower specimen, tufftrided and carburized steels demonstrate similar wear behavior, as do the TiN and CrN coated steels_ SKHsl paired with carburized steel under cutting fluid lubrication demonstrates the Ieast wear, exhibiting only surface polishing (no measurable wear). In Fig. S(b), note that under either cutting fluid or base oil lubrication, the PVD CrN coated steel specimen mated with either TiN or CrN PVD coated counterparts possesses the best wear resistance. Under dry wear conditions, however, CrN mated with PVD coated counterparts exhibits higher wear, in the form of deeper scar depths, than when mated with surface treated steels. Thus, it is concluded that PVD-PVD sliding pairs possess the best wear resistance, but only under lubricated conditions. In Fig. 8(c) note that, under dry wear conditions, TiN+CrN paired with CrN exhibits excessive wear, much higher than with TiN. This behavior may arise from the high chemical affinity of self-mated materials for each other, which induces material adhesion. As in Fig. S(a), cutting fluid lubrication yields the best wear performance,
49
Y.L. Su, J.S. Lin I Tribologkal potential of PVD coatings for machine elements TABLE 2. The tribological performance of the various sliding pairs studied under measurable wear; “base” = base oil lubrication; “cutting” = cutting fluid lubrication Upper specimen
CARB
TFl
TiN (3 pm) CrN (LO pm)
TiN (3 pm)+CrN
different
lubricating
(10 pm)
conditions:
“polish”=no
CrN (10 pm)
Lower specimen environment
sKH51 Upper specimen width (mm)
Lower specimen depth (pm)
Upper specimen width (mm)
Lower specimen depth (wm)
Upper specimen width (mm)
Lower specimen depth (pm)
DrY Base Cutting DIy Base Cutting DrY Base Cutting DIy Base Cutting
1.31 0.43 0.44 1.38 0.56 0.42 0.90 0.35 0.24 0.76 0.38 0.23
26.9 3.54 Polish 23.80 4.00 0.23 20.83 0.95 0.45 19.67 0.73 0.20
0.96 0.65 0.57 1.40 0.74 0.67 0.60 0.23 0.25 0.71 0.32 0.30
1.69 14.78 0.02 1.62 14.1 Polish 0.63 Polish Polish 11.78 Polish Polish
0.85 0.60 0.41 1.07 0.50 0.57 0.73 0.20 0.24 0.42 0.21 0.25
1.02 4.80 Polish 0.81 7.00 Polish 5.25 Polish Polish 10.00 Polish Polish
Three types of friction curve were obtained from the wear testing results (see Fig. 9(a)). A typical example of each is shown in Fig. 9(b). The type 3 friction curve originates under dry wear conditions and is characterized by high friction that fluctuates over a narrow range. Type 1 and 2 originate under lubricated conditions and are divided into two types: “short” and “long” run-in respectively. The latter is exhibited by surface treated specimens-PVD coated specimens under base oil lubrication; the former is exhibited by surface treated steel-PVD coated specimens and PVD-PVD pairs under cutting fluid lubrication. The “long” run-in is defined as a longer time of initial material surface asperity contact, which induces intermittent scuffing. This behavior is not desirable. The coefficient of friction under dry wear conditions is approximately six to seven times larger than under lubrication, reflecting the fact that lubrication definitely improves wear performance.
Fig. 11. In Fig. 11(a), the worn surface is covered with a thin film of back-transferred material; in Fig. 11(b) the worn surface is highly spalled due to the hard CrN surface. In Fig. 11(c), the worn surface exhibits only fine polishing grooves. The worn surface of a tufftrided specimen that had been paired with a CrN coated specimen under base oil lubrication is shown in Fig. 12(a). The wear mechanism here is spalling due to surface fatigue. The wear process can be described by crack connection inside the work hardened layer. As shown under higher magnification (Fig. 12(b)), the direction of crack propagation is not regular, with most of the cracks being parallel or perpendicular to the sliding direction (horizontal direction); these results are characteristic of fatigue wear.
3.4. Mechanisms of wear
The worn surface of a CrN coated specimen that had been paired with a tufftrided specimen under dry wear conditions is shown in Fig. 13(a). The surface is covered with a transferred layer of iron. When CrN is self-mated under dry wear conditions, due to high surface hardness of these PVD coated parts and the strong adhesion of self-mated sliding pairs, the wear depth of CrN-CrN is extremely high (see Fig. 13(b)). The worn surface of a CrN coated specimen that had been paired with a tufftrided specimen under base oil lubrication is shown in Fig. 13(c). The surface is highly damaged, and so is that of the mated tufftrided specimen (refer to Figs. 12(a) and 12(b)). Apparently, when paired with upper moving element surface treated steels, CrN and TiN+CrN PVD coated parts possess
3.4.2. PVLI coated specimen (TiN, CrN or TiN + CrN)
3.4.1. Sulfate treated steels (CAREI or TFl) Sk7751
and
The worn surface of an SKI-El (HRC 62) specimen that had been paired with a tufftrided specimen under dry wear conditions is shown in Fig. 10. In the wear scar region, the worn surface is covered with a thin transferred layer film, composed of mainly Fe with some Fe oxide particles (Fe,O, and Fe,O,). Since the wear debris is thin and irregular, its removal from the surface is attributed to spalling of the transferred material. The worn surface of carburized specimens paired with CrN coated specimens under dry wear conditions and base oil and cutting fluid lubrication is shown in
A:CARB-SKHkky) B:CARB-SKH(btw) C:CARESKH~IO D:TFl-SKH(&y) E:TF-SKHbase) FTFI-SKH(cut) c:TlN-SKH(dry) H:TlN-SKH&w) I:TiN-SKHkuO J:CrN-SKH(dry)
K:CrN-SKH(base) ~CrN-SKH(cul) 0.0 0.2 (a)
0.4
0.6 0.8
1.0 1.2
: 0.6 8 0.4 2 0.2
::
._
0
300
600
900
1200
1.500
TIME (SECONDS)
(a)
1.4 1.6
Upper specimen wear width(mm)
q
A:CARBCRN(dly~ BCARB-CRNW)
DRY
A BASEOIL 0
culTrNcFLurD
Time
c:cARB-cRN(cut)
(b)
D:TFl-CRN(dl)+
Fig. 9. (a) The typical friction types of CrN (10 pm) specimens paired with different surface treated steels: (1) TiN coated specimen paired with CrN (cutting fluid); (2) carburized paired with CrN (base oil); (3) carburized paired with CrN (dry wear). fb) The friction curve concluded for various CrN shding pairs: (1) short “running-in”, lubricated wear; (2) long “running-in”, lubricated wear; (3) dry wear.
E:TFWRN@ase) F:TFl-CRN(cut) (YmfwJw@) H:TIN-CRN~) I:~~(cut) I:~~(~) K:C~~~~) L:CRN-CRi?(cutf
0.0 0.2
0))
0.4 0.6 0.8 1.0 1.2 1.4 1.6 Upper specimen wear width(mm)
L:CRN-TTNCRNW)
0.0 0.2 0.4 0.6 0.8
Cc)
1.0 1.2
1.4 1.6
Fig. 10. The worn surface of an SKH51 (HRC62) specimen that had been paired with a tufftrided specimen under dry wear conditions.
Upper specimen wear width(mm)
Fig. 8. (a) The wear of (a) SKH51 samples, (b) CrN (10 grn) coated samples and (c) TiN (3 pm) + CrN (10 pm) coated samples, (all lower specimens) that had been paired with various surface treated and PVD coated steels under dry and lubricated conditions.
very poor wear resistance under base oil lubrication, even worse than under dry wear conditions. The worn surface of a CrN coated specimen that had been paired with a CrN coated specimen under cutting fluid lubrication is shown in Fig. 13(d). Note that there is no evidence of wear, with only surface polishing being visible. In fact, all other PVD-PVD pairs demonstrated surface polishing under base oil lubrication. PVD-PVD sliding pairs appear to possess good wear resistance under cutting fluid or base oil lubrication.
The worn surface of a TiN (3 pm) + CrN (10 pm) coated specimen that had been paired with a tufftrided specimen under base oil lubrication is shown in Fig. 14(a); its chromium and iron X-ray maps are shown in Figs. 14(b) and (c ) , respectively. Although the scar depth examination results (depth = 14.1 pm) indicate that the surface TiN +CrN PVD coated layer should have been worn away before the experiment concluded, some residual CrN and TiN remains on the worn surface. The Fe X-ray map reveals the presence of the exposed SKI%1 substrate. 3.5. Wear debris The wear debris of a TiN coated specimen that had been paired with an SKII specimen under dry wear conditions is shown in Fig. 15(a). The wear debris
XL. Su, J.S. Lin / T~l~.ca~
patently
of PVT, coatings for machine elements
51
(4 Fig. 11. The worn surface of a carburized specimen that had been paired with a CrN coated specimen (10 JUII): (a> dry wear; (b) base oil lubrication; (c) cutting fluid lubrication.
Fig. 13. The worn surface of a CrN (10 pm) coated specimen: (a) paired with a tufftrided specimen, dry wear; (b) paired with a CrN coated specimen, dry wear; (c) paired with a tufftrided specimen, base oif; (d) paired with a CrN coated specimen, cutting fluid.
09 Fig. 12. The worn surface of a tufFtrided specimen that had been paired with a CrN (10 pm) coated specimen under base oil lubrication: (a) worn surface; (b) 5 X magnification of the specimen shown in (a).
results from delamination of the transfer layer, and shows fine “network-like” cracks on its surface. Both these wear characteristics are attributed to fatigue crack propagation as a result of repeated contact stress. The wear debris of a CrN coated specimen that had been paired with a CrN coated specimen under dry wear conditions is shown in Fig. 15(b). Note that the wear debris also result from deIamination due to internal cracking of the surface coating [18].
4. Discussion The five best sliding pair systems (upper-lower) in terms of least wear are: TiN (3 pm)-CrN (10 pm), base oil > CrN (10 pm)-CrN (10 pm), base oil> TiN (3 pm)-TiN (3 pm)/CrN (10 pm), base oil>TiN (3 pm)-CrN (10 pm), cutting fluid> CrN (10 pm)-CrN (10 pm), cutting &rid. The five worst sliding pair systems
Fig. 14. The worn surface of a TiN (3 pm) f CrN (10 pm) coated specimen that had been paired with a tufftrided specimen under base oil lubrication: (a) worn surface; (b) Cr X-ray map; (c) Fe X-ray map.
(upper-lower) in terms of most wear are: carburized-SKH51, dry wear > TFl-SKH51, dry wear > TiN (3 ~m)-SKH51, dry wear> CrN (10 pm)-SKH51, dry wear>TFl-TiN (3 pm)/CrN (10 pm), base oil. All other sliding pair systems studied fall somewhere between the pairs listed above. Each sliding pair system is discussed in general terms beiow (see Table 2 and Fig. 8).
52
Y.L. Su, J.S. Lin I Tribological potential of PVD coatings for machine elements TABLE 3. Tribological performance Sliding pairs
_
(a)
_ .-__
@I
Fig. 15. (a) The wear debris from pm) that had been paired with an wear conditions; (b) the wear debris (10 pm) that had been paired with pm) under dry wear conditions.
a TiN coated specimen (3 SKH51 specimen under dry from a CrN coated specimen a CrN coated specimen (10
4.1. Surface treated steels (CARB or TFl-SKH51)
Under dry wear conditions, CARB or TFl specimens mated with SKH51 specimens possess the worst wear resistance of all the sliding pairs. Wear resistance is remarkably improved under lubricated wear conditions, with cutting fluid yielding the best performance. 4.2. Su$ace treated steels (CARB or TFl)-PYD (CrN or TiN+ CrN)
coated
Under base oil lubrication, both surface treated steels and PVD coated parts possess very poor wear resistance, even worse than under dry wear conditions. On the other hand, water based cutting fluid lubrication is very suitable for these sliding pairs, with only polishing being exhibited in the lower specimen in most cases. Note that under both dry and lubricated wear conditions, the carburized steel and tufftrided specimens exhibit similar wear resistance. The poor wear resistance of surface treated steels paired with PVD coated specimens may be due to the higher coefficient of friction (see Fig. 9(a)) inducing higher contact stress, causing both sides of the sliding surfaces to show fatigue cracking and spalling (see Figs. 11(b), 12(a) and 13(c)); the spalled material will interfere with the lubrication and widen the scope of failure. 4.3. PKD coated specimens (TiN or CrN)-Sk7951
This sliding pair possesses poor wear resistance under dry wear; the wear scar width of specimens is much lower than that of surface treated steels paired with SKHSl. This is due to the higher hardness of PVD coated specimens. The wear resistance under cutting fluid lubrication is better than under base oil lubrication. 4.4. PKD coated specimens (TiN or CrN)-PVD
coated
(CrN or TiN+ CrN)
Although poor wear resistance is obtained under dry wear conditions, especially for CrN-TiN + CrN and CrN-CrN pairs, PVD-PVD sliding pairs possess good
ofvarious sliding pairs studied
Performance
Cutting fluid
Dry
Base oil
CARB-SKHS 1 Tufftrided-SKH51
Bad
Adequate
Excellent
TiN-SKH51 CrN-SKH5 1
Bad
Adequate
Excellent
CARB-TIN + CrN CARB-CrN Tufftrided-TiN + CrN Tufftrided-CrN
Adequate
Bad
Excellent
TiN-TiN + CrN TiN-CrN CrN-TiN + CrN CrN-CrN
Bad
Excellent
Excellent
wear resistance under cutting fluid or base oil lubrication. Poor performance under dry wear conditions is thought to arise from strong adhesion and the relatively high degree of hardness of self-mated sliding pairs. The dual TiN +CrN coating demonstrates no better wear resistance than a single layer of CrN coating. The wear performance of the four sliding pairs mentioned above is summarized in Table 3. 4.5. Suitable sliding pair for VLSTM The following sliding (upper-lower) pairs are suitable for VLSTM applications: (1) surface treated steels paired with SKHSl, TiN+CrN or CrN PVD coated specimens under cutting fluid lubrication; (2) TiN or CrN PVD coated specimens paired with SKH51 specimens under cutting fluid lubrication; (3) TiN or CrN PVD coated specimens paired with TIN + CrN or CrN PVD coated specimens under base oil and cutting fluid lubrication.
5. Conclusions (1) Sliding pairs generally possessed the best wear resistance under cutting fluid lubrication. Although surface treated steels paired with CrN or TiN+CrN PVD coated parts exhibited very poor wear resistance under base oil lubrication, even worse than under dry wear conditions, wear performance was remarkably better under cutting fluid lubrication. (2) PVD coated specimens paired with surface treated steels under dry wear conditions exhibited a transferred layer composed mostly of iron. (3) Under cutting fluid lubrication, the worn surface of PVD lower specimens consistently exhibited surface polishing (no measurable wear). These results were independent of upper pair choice.
XL. Su, J.S. Lin I Tribological potential of PVD coatings for machine elements
(4) Suitable sliding pairs for VLSTM applications include: surface treated steels paired with either SIMS1 or PVD coated specimens under cutting fluid lubrication; PVD coated specimens paired with SKH51 specimens under cutting fluid lubrication; and PVD coated specimens paired with PVD coated specimens under base oil and cutting fluid lubrication.
Acknowledgments The authors are grateful to the National Science Council of the Republic of China for supporting this research under grant NSC79-0414-EOO6-105R. This project was directed by Dr. Hong-Sen Yan and Dr. Rong-Shean Lee.
References
53
4 C.T. Young and S.K. Rhee, Wear processes of TiN-coated drills, Proc. ASMEISTLE Joint Symp. on Wear of Materials, Houston, TX, 1987, pp. 543-550. 5 W.D. Sproul, Thin Solid Films, 26 (1985) 257-263. 6 Z. Palmai, Wear, 95 (1984) l-7. 7 F.A. Soliman and O.A. Abu-Zeid, Wear, I19 (1987) 199-204. 8 H. Randhawa, J. Vat. Sci. TechnoZ., A4 (6) (1986) 2755-2758. 9 R. Buhl, H.K. Pulker and E. Moll, Thin Solid Films, 80 (1981) 265-270. 10 A.K. Chattopadhyay and A.B. Chattopadhyay, Wear, 80 (1982) 239-258. 11 M.J. Park, A. Leyland and A. Matthews, Surf: Coat. Technol., 43144 (1990) 481492. 12 E. Rabinowicz, ASLE Trans., 14 (1971) 198-205. 13 Y.L. Su, J.S. Lin, L.I. Shiau and J.D. Wu, Wear, 167 (1993) 73-83. 14 Development and Practical Application of the Tz@rided Process (Old and New). Degussa company technical information, Hanau, Germany, 1975. 15 T. Arai, H. Fujita and M. Watanabe, Thin Solid Films, 154 (1987) 387-401. between 16 S.J. Bull and D.S. Rickerby, The inter-relationship coating microstructure and the tribological performance of PVD coatings, in D. Dowson, C.M. Taylor and M. Godet (eds.), Mechanics of Coatings, Proc. 16th Leeds-Lyon Symp. on Tkbolog~, 1989, pp. 3371348. 17 0. Knotek. F. Loffler and H.J. Scholl. , Surt: Coat. Technol.. 45 (1991) 53-58. of 18 A.W. Ruff, L.K. Ives and W.A. Glaeser, Characterization wear surface and wear debris, in D.A. Rigney (ed.), Fundamentals of Friction and Wear of Materials, American Society for Metals, Metals Park, OH, 1981, pp. 235-289. ”
1 L. Pezzoli, United States Patent 4 624 288, 1986. 2 Y.L. Su and J.S. Lin, Wear, 166 (1993) 27-35. 3 Y.L. Su and J.S. Lin, Wear, 160 (1993) 139-151.
Wear, 170 (1993) 45-53
investigation .of the tribological potential of TiN, CrN and TiN+ CrN physical vapor deposited coatings in machine element applications
An
Y.L. Su and J.S. Lin Department of Mechanical Engineering, National Cheng Kung lJniversi& Tainan 70101 (Taiwan) (Received
February
22, 1993; accepted
June 24, 1993)
Abstract This study investigates, through wear testing, the potential for carburized and tufftrided surface treated steels and the TiN, CrN and TiN+ CrN physical vapor deposited (PVD) surface treatments to be used in the screw and rollers of the variable lead screw transmission mechanism (VLSTM). Indentation test results reveal that the thicker the PVD CrN coating, the lower is its adhesive strength, with the desired coating thickness being 10 pm. In general, all sliding pairs possess the best wear resistance under cutting fluid lubrication. Surface treated steels (upper moving specimen) paired with PVD coated parts possess very poor wear resistance under base oil lubrication, even worse than under dry wear. Suitable sliding pairs for VLSTM applications include: surface treated steels paired with either SKH51 or PVD coated specimens under cutting fluid lubrication; PVD coated specimens paired with SKH51 specimens under cutting fluid lubrication; and PVD coated specimens paired with PVD coated specimens under base oil and cutting fluid lubrication.
1. Introduction The VLSTM [l] is the heart of “shuttleless” textile machinery, and possesses four rollers driven by a fourbar linkage mechanism. The rollers slide against the screw, with the resulting angular accelerating motion of the latter being used as output (Figs. 1 and 2). The problem associated with this mechanism arises from contact loading and the sliding velocity inducing wear in both the screw and rollers. Previous work in this laboratory [2,3] addressed the high wear rate problem by considering ceramic coated and surface treated steels for the rollers and surface treated steels for the screws. PVD nitride and carbide coatings are being increasingly employed in both tribological and corrosion re-
ariable lead screw
Belt whiel
Fig. 1. A schematic
0043-1648/93/$6.00
diagram of the VLSTM.
iable pitch cylindrical
cam
Fig. 2. The variable lead screw is driven by four rollers (3a, 3b, 3c and 3d).
sistant applications [4-6], especially in the metal cutting industry [7-111. The scarcity of work done on TiN, CrN and TiN+ CrN coatings suggests possible applications in machine elements. This study investigates, through wear testing, the potential for the TiN, CrN and TiN + CrN surface treatments to be used in VLSTM screws and rollers. Carburized and tufftrided surface treated steels are also examined. The criterion of compatibility [12] is one of the most important principles in tribological material selection. Suitably mated materials can not only reduce the wear
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of the PVD coated part itself, but also reduce wear in the counterpart. Since TiN and CrN coatings are quite hard, an excessively soft counterpart will easily degrade. Obviously, the choice of a suitable sliding pair warrants careful consideration and further study.
2. Experimental method The compositions of the three materials (JIS standard) used in these experiments are listed in Table 1. The SKII is a high speed steel. The SACM645 contains aluminum, which can form aluminum nitride during nitridation. The SCM415 is a low carbon steel for carburization application. 2.1. Specimen preparation 2.1.1.
TiN coating
Prior to deposition, SKI-I51 (JIS standard) substrates were vacuum heat treated (quenched at 1220 “C, followed by tempering at 560 “C) to HV763 characteristics, and then polished with 0.3 pm A&O, powder to a roughness Rz of less than 1.2 ,um. Titanium nitride PVD coatings were deposited on SISH51 substrates through the use of a multiple cathode arc plasma (CAP) system. Substrate temperatures during deposition were maintained at 400-450 “C. The processing parameters (- 100 V substrate bias voltage, 80 A arc current, 10m2 mbar N, partial pressure, 3,um coating thickness) represent optimal conditions determined previously by the authors [13]. The microhardness (HV25g) of PVD TiN 3pm thick is 2398. 2.1.2. CrN and TiN~3~~ + CrN coatings Chromium nitride PVD coatings were also deposited on SKH51 substrates using a CAP system (Multi-arc Company, USA), Substrate temperatures during deposition were maintained at 400 “C. Processing parameters similar to the above, except that NZ partial pressure was increased to 0.26 mbar, were employed. Optimum coating thickness was determined through dry wear testing of 5 @urn,10 Mm and 15 pm CrN coatings paired with tufftrided specimens.
SACM645 base materials were quenched and tempered to HRC30 characteristics. The tufftrided trcatmcnt is a salt bath nitriding process [ 141 that induces an I-IV1018 surface microhardness to SACM645. A bath temperature of 580 “C and a 3 h treatment time were employed in the tufftriding process. Thermally refined SCM415 (HRC30) was carburized to HRC60 surface hardness. 2.2. Wear testing Wear testing was performed by an SRV oscillating wear machine produced by the Optimol Company, Germany. The coefficient of friction, load, stroke and temperature were recorded in real time by a computerized data acquisition system (DAS). The dimensions of the test specimens and a schematic diagram of the experimental setup are shown in Fig. 3. Note that the upper cylindrical specimen is in contact with the flat side of the lower specimen, and that the former oscillates horizontally in the axial direction. The tests were conducted at room temperature and atmospheric pressure. The relative humidity of the laboratory was approximately 45%-50%. Constant test strokes, normal loads and frequencies of 0.5 mm, 300 N and 50 Hz respectively were employed. Specimens were first run in at 20 N and 30 Hz for 2 min, with the frictional data not being recorded. The total sliding distance under both dry and lubricated wear conditions was approximately 72 m. Base oil and synthetic oil were the lubricants used in this study. The former possesses kinematic viscosities
Oscillating Driving motor Force transducer
2.1.3, S.w$ace treated ~~e&irnen~ Tufftridation and carburization were the surface treatments. Prior to surface treatment, the SCM415 and TABLE
1. Specimen
composition
L
(a)
,1,
recipmcating
(%) (a)front view
Si
MnP
1
S
Al
Ni
Material
C
Cr
MO Fe
SACM645 SCM415 SW51
0.46 0.23 0.5 0.02 0.018 0.8 0.050 1.5 0.2 0.17 0.25 0.69 0.02 0.018 0.037 1.0 0.2 0.85 0.23 0.3 0.02 0.007 0.06 4.1 5.3
Bal. Bal. Bal.
(b)
n upper q Lower
(b)side view
specimen(diameter:lJmm
lengthz22mm)
specimen(diameter:24mm
height:7.9mmf
Fig. 3. (a) A schematic specimen dimensions.
diagram
of the experiment
setup;
(b)
XL. Su, J.S. Lin f Tribdogicai
pote?ztiai of PVD coatings for machine elements
of 138.9 cSt and 14.4 cSt at 40 “C and 100 “C respectively. The latter is a water-based cutting fluid ~ynn’s 983) devoid of sulphur and chlorine. Its specific gravity is 1.018 at 15 “C; the pH value of a 1:20 oil/water solution (diluted for application purposes} is 8.9-9.2. This is typical for medium and heavy cutting applications. Dry wear was also investigated for comparison with lubricated wear performance. 2.3. Related equipment A profilometer was used to measure specimen wear scar dimensions. The amount of wear is evaluated based on wear scar depth for the lower specimen (SKI%1 substrates with and without subsequent CrN or TiN+CrN coatings) and wear scar width for the upper specimen (surface treated steels, TiN and CrN coated specimens). Microhardness testing was used to examine specimen surface and subsurface microhardness. Scanning electron microscopy (SEM) was used for metallographic examination and worn surface observation; X-ray diffraction was employed in identi~ng the preferred orientation of the PVD coatings. Rockwell C scale indentation tests (150 kg), performed on CrN coated specimens of different coating thicknesses, helped to determine the adhesive strength of these PVD coatings 1151.
47
_---.
(a)
0’)
(cl Fig. 4. (a) Microstructure of a tufftrided treated TF-1 specimen; (b) line-scan of Ti distribution for TiN (3 pm) PVD coating; (c) line-scan of Cr distribution for CrN (10 Frn)PVD coating.
3. Results 3.1. Microstructureexamination The microstructure of a tufftride treated specimen is shown in Fig. 4(a), where the secondary electron image (SEI) is on the left and the back scattered electron image (BEI) is on the right. This surface treatment yields a porous r-Fe,N single phase surface zone, consisting of iron, nitrogen and carbon compounds. In addition to good anti-frictional and wear behavior, this compound layer exhibits no tendency to seize. In both electron images, the “tree” structure in the subsurface region of the sample is y’-Fe,N. The surface mo~holo~ of PVD TiN (3 ,um) is shown in Fig. 4(b). Note the presence of many irregularly shaped droplets on the coating surface. The Ti linescan clearly indicates that these particles are titanium rich, which, unfortunately, decreases the coating’s anticorrosion performance and wear resistance under severe wear conditions [16]. Note that the surface morphology of PVD CrN (10 pm) shown in Fig. 4(c) is similar to that of TiN discussed above. The Cr line-scan, in contrast, indicates that the particles are not chromium-rich, being of the same composition as the CrN surface coating. Although no cross-sectional studies of these droplets were made, these particles may well be chromium rich at their core.
(Cl Fig. 5. The indentation cracking of various CrN coating thicknesses: (a) 5 pm; (b) 10 pm; (G) 15 pm.
The results of CrN wear tests revealed that droplets decrease wear resistance, especially during the runningin phase. Note, however, that the anticorrosive behavior of arc-evaporated CrN coatings can still be good [17]. 3.2, Effect of coating thickness Photographs of 5, 10 and 15 pm thick CrN coatings after the Rockwell hardness C scale indentation test (load 150 kg) are shown in Fig. 5. Each of the three CrN coatings possesses radial cracks, and that lateral
cracks becume more visible with an increase in coating thickness. These results indicate that the brittleness of the coating layer increases under high internal stress, while the adhesive strength decreases with an increase in coating thickness. The surface texture of PVD CrN (see Fig. 6) consists only of the (220) phase, while PVD TiN consists of the (111) and (222) phases [13]. These preferred orientations do not alter as the coating thickness is increased. The mi~rohardn~ss of CrN coating with various coating thickness is shown in Fig. 7(a). It seems that as the coating thickness is increased, the microhardness is increased. This is attributed to either the substrate effect at the smaller coating thickness or the increase in internal stresses at greater thickness, as shown in Fig. 5. The relationship between coating thickness and wear depth of CrN coated specimens paired with tufftrided specimens under dry wear conditions is shown in Fig. 7(b). In terms of optimal wear resistance, the CrN coating thickness should be approximately 10 pm. If the coating thickness is 15 pm, although it possesses good wear resistance, the brittleness of the coating layer is increased (as described above). If the coating is too thin (5 pm), it will be easily worn through. 3.3. Wear and fictkm The wear testing results of carburized, tufftrided, TiN (3 pm) and CrN (10 pm) upper specimens paired with SKJSl, TiN (3 pm)/CrN (10 pm) and CrN (10 pm) lower specimens under both dry and lubricated conditions are given in Table 2, In order to ease the inte~r~tation of these results, wear depths of the SKH51, TiN + CrN and CrN lower specimens are plotted --I
t
CrN(ZZ0)
Fig. 6. X-ray diffraction patterns of SIG351 samples with and without subsequent TiN or CrN coatings of different thicknesses: (a) SKEEI; (b) CrN, 5 ,um; (c) CrN, 10 ,um; fd) CrN, 15 pm.
Substrate
IOum
Sum
15um
Coatinp. thickness
(a)
5
10 FiLM
15
TWiCKNESS(um)
Fig. 7. {a) The relation of coating thickness and microhardness of CrN coated samples with various thicknesses (HV25g); (b) the relation of coating thickness and wear depth for CrN coated samples (upper specimens) paired with tufftrided samples (lower specimens) under dry wear conditions.
against the wear scar width of their various upper specimen pairs in Fig. 8, In Fig. S(a), where SKIS1 is the lower specimen, tufftrided and carburized steels demonstrate similar wear behavior, as do the TiN and CrN coated steels_ SKHsl paired with carburized steel under cutting fluid lubrication demonstrates the Ieast wear, exhibiting only surface polishing (no measurable wear). In Fig. S(b), note that under either cutting fluid or base oil lubrication, the PVD CrN coated steel specimen mated with either TiN or CrN PVD coated counterparts possesses the best wear resistance. Under dry wear conditions, however, CrN mated with PVD coated counterparts exhibits higher wear, in the form of deeper scar depths, than when mated with surface treated steels. Thus, it is concluded that PVD-PVD sliding pairs possess the best wear resistance, but only under lubricated conditions. In Fig. 8(c) note that, under dry wear conditions, TiN+CrN paired with CrN exhibits excessive wear, much higher than with TiN. This behavior may arise from the high chemical affinity of self-mated materials for each other, which induces material adhesion. As in Fig. S(a), cutting fluid lubrication yields the best wear performance,
49
Y.L. Su, J.S. Lin I Tribologkal potential of PVD coatings for machine elements TABLE 2. The tribological performance of the various sliding pairs studied under measurable wear; “base” = base oil lubrication; “cutting” = cutting fluid lubrication Upper specimen
CARB
TFl
TiN (3 pm) CrN (LO pm)
TiN (3 pm)+CrN
different
lubricating
(10 pm)
conditions:
“polish”=no
CrN (10 pm)
Lower specimen environment
sKH51 Upper specimen width (mm)
Lower specimen depth (pm)
Upper specimen width (mm)
Lower specimen depth (wm)
Upper specimen width (mm)
Lower specimen depth (pm)
DrY Base Cutting DIy Base Cutting DrY Base Cutting DIy Base Cutting
1.31 0.43 0.44 1.38 0.56 0.42 0.90 0.35 0.24 0.76 0.38 0.23
26.9 3.54 Polish 23.80 4.00 0.23 20.83 0.95 0.45 19.67 0.73 0.20
0.96 0.65 0.57 1.40 0.74 0.67 0.60 0.23 0.25 0.71 0.32 0.30
1.69 14.78 0.02 1.62 14.1 Polish 0.63 Polish Polish 11.78 Polish Polish
0.85 0.60 0.41 1.07 0.50 0.57 0.73 0.20 0.24 0.42 0.21 0.25
1.02 4.80 Polish 0.81 7.00 Polish 5.25 Polish Polish 10.00 Polish Polish
Three types of friction curve were obtained from the wear testing results (see Fig. 9(a)). A typical example of each is shown in Fig. 9(b). The type 3 friction curve originates under dry wear conditions and is characterized by high friction that fluctuates over a narrow range. Type 1 and 2 originate under lubricated conditions and are divided into two types: “short” and “long” run-in respectively. The latter is exhibited by surface treated specimens-PVD coated specimens under base oil lubrication; the former is exhibited by surface treated steel-PVD coated specimens and PVD-PVD pairs under cutting fluid lubrication. The “long” run-in is defined as a longer time of initial material surface asperity contact, which induces intermittent scuffing. This behavior is not desirable. The coefficient of friction under dry wear conditions is approximately six to seven times larger than under lubrication, reflecting the fact that lubrication definitely improves wear performance.
Fig. 11. In Fig. 11(a), the worn surface is covered with a thin film of back-transferred material; in Fig. 11(b) the worn surface is highly spalled due to the hard CrN surface. In Fig. 11(c), the worn surface exhibits only fine polishing grooves. The worn surface of a tufftrided specimen that had been paired with a CrN coated specimen under base oil lubrication is shown in Fig. 12(a). The wear mechanism here is spalling due to surface fatigue. The wear process can be described by crack connection inside the work hardened layer. As shown under higher magnification (Fig. 12(b)), the direction of crack propagation is not regular, with most of the cracks being parallel or perpendicular to the sliding direction (horizontal direction); these results are characteristic of fatigue wear.
3.4. Mechanisms of wear
The worn surface of a CrN coated specimen that had been paired with a tufftrided specimen under dry wear conditions is shown in Fig. 13(a). The surface is covered with a transferred layer of iron. When CrN is self-mated under dry wear conditions, due to high surface hardness of these PVD coated parts and the strong adhesion of self-mated sliding pairs, the wear depth of CrN-CrN is extremely high (see Fig. 13(b)). The worn surface of a CrN coated specimen that had been paired with a tufftrided specimen under base oil lubrication is shown in Fig. 13(c). The surface is highly damaged, and so is that of the mated tufftrided specimen (refer to Figs. 12(a) and 12(b)). Apparently, when paired with upper moving element surface treated steels, CrN and TiN+CrN PVD coated parts possess
3.4.2. PVLI coated specimen (TiN, CrN or TiN + CrN)
3.4.1. Sulfate treated steels (CAREI or TFl) Sk7751
and
The worn surface of an SKI-El (HRC 62) specimen that had been paired with a tufftrided specimen under dry wear conditions is shown in Fig. 10. In the wear scar region, the worn surface is covered with a thin transferred layer film, composed of mainly Fe with some Fe oxide particles (Fe,O, and Fe,O,). Since the wear debris is thin and irregular, its removal from the surface is attributed to spalling of the transferred material. The worn surface of carburized specimens paired with CrN coated specimens under dry wear conditions and base oil and cutting fluid lubrication is shown in
A:CARB-SKHkky) B:CARB-SKH(btw) C:CARESKH~IO D:TFl-SKH(&y) E:TF-SKHbase) FTFI-SKH(cut) c:TlN-SKH(dry) H:TlN-SKH&w) I:TiN-SKHkuO J:CrN-SKH(dry)
K:CrN-SKH(base) ~CrN-SKH(cul) 0.0 0.2 (a)
0.4
0.6 0.8
1.0 1.2
: 0.6 8 0.4 2 0.2
::
._
0
300
600
900
1200
1.500
TIME (SECONDS)
(a)
1.4 1.6
Upper specimen wear width(mm)
q
A:CARBCRN(dly~ BCARB-CRNW)
DRY
A BASEOIL 0
culTrNcFLurD
Time
c:cARB-cRN(cut)
(b)
D:TFl-CRN(dl)+
Fig. 9. (a) The typical friction types of CrN (10 pm) specimens paired with different surface treated steels: (1) TiN coated specimen paired with CrN (cutting fluid); (2) carburized paired with CrN (base oil); (3) carburized paired with CrN (dry wear). fb) The friction curve concluded for various CrN shding pairs: (1) short “running-in”, lubricated wear; (2) long “running-in”, lubricated wear; (3) dry wear.
E:TFWRN@ase) F:TFl-CRN(cut) (YmfwJw@) H:TIN-CRN~) I:~~(cut) I:~~(~) K:C~~~~) L:CRN-CRi?(cutf
0.0 0.2
0))
0.4 0.6 0.8 1.0 1.2 1.4 1.6 Upper specimen wear width(mm)
L:CRN-TTNCRNW)
0.0 0.2 0.4 0.6 0.8
Cc)
1.0 1.2
1.4 1.6
Fig. 10. The worn surface of an SKH51 (HRC62) specimen that had been paired with a tufftrided specimen under dry wear conditions.
Upper specimen wear width(mm)
Fig. 8. (a) The wear of (a) SKH51 samples, (b) CrN (10 grn) coated samples and (c) TiN (3 pm) + CrN (10 pm) coated samples, (all lower specimens) that had been paired with various surface treated and PVD coated steels under dry and lubricated conditions.
very poor wear resistance under base oil lubrication, even worse than under dry wear conditions. The worn surface of a CrN coated specimen that had been paired with a CrN coated specimen under cutting fluid lubrication is shown in Fig. 13(d). Note that there is no evidence of wear, with only surface polishing being visible. In fact, all other PVD-PVD pairs demonstrated surface polishing under base oil lubrication. PVD-PVD sliding pairs appear to possess good wear resistance under cutting fluid or base oil lubrication.
The worn surface of a TiN (3 pm) + CrN (10 pm) coated specimen that had been paired with a tufftrided specimen under base oil lubrication is shown in Fig. 14(a); its chromium and iron X-ray maps are shown in Figs. 14(b) and (c ) , respectively. Although the scar depth examination results (depth = 14.1 pm) indicate that the surface TiN +CrN PVD coated layer should have been worn away before the experiment concluded, some residual CrN and TiN remains on the worn surface. The Fe X-ray map reveals the presence of the exposed SKI%1 substrate. 3.5. Wear debris The wear debris of a TiN coated specimen that had been paired with an SKII specimen under dry wear conditions is shown in Fig. 15(a). The wear debris
XL. Su, J.S. Lin / T~l~.ca~
patently
of PVT, coatings for machine elements
51
(4 Fig. 11. The worn surface of a carburized specimen that had been paired with a CrN coated specimen (10 JUII): (a> dry wear; (b) base oil lubrication; (c) cutting fluid lubrication.
Fig. 13. The worn surface of a CrN (10 pm) coated specimen: (a) paired with a tufftrided specimen, dry wear; (b) paired with a CrN coated specimen, dry wear; (c) paired with a tufftrided specimen, base oif; (d) paired with a CrN coated specimen, cutting fluid.
09 Fig. 12. The worn surface of a tufFtrided specimen that had been paired with a CrN (10 pm) coated specimen under base oil lubrication: (a) worn surface; (b) 5 X magnification of the specimen shown in (a).
results from delamination of the transfer layer, and shows fine “network-like” cracks on its surface. Both these wear characteristics are attributed to fatigue crack propagation as a result of repeated contact stress. The wear debris of a CrN coated specimen that had been paired with a CrN coated specimen under dry wear conditions is shown in Fig. 15(b). Note that the wear debris also result from deIamination due to internal cracking of the surface coating [18].
4. Discussion The five best sliding pair systems (upper-lower) in terms of least wear are: TiN (3 pm)-CrN (10 pm), base oil > CrN (10 pm)-CrN (10 pm), base oil> TiN (3 pm)-TiN (3 pm)/CrN (10 pm), base oil>TiN (3 pm)-CrN (10 pm), cutting fluid> CrN (10 pm)-CrN (10 pm), cutting &rid. The five worst sliding pair systems
Fig. 14. The worn surface of a TiN (3 pm) f CrN (10 pm) coated specimen that had been paired with a tufftrided specimen under base oil lubrication: (a) worn surface; (b) Cr X-ray map; (c) Fe X-ray map.
(upper-lower) in terms of most wear are: carburized-SKH51, dry wear > TFl-SKH51, dry wear > TiN (3 ~m)-SKH51, dry wear> CrN (10 pm)-SKH51, dry wear>TFl-TiN (3 pm)/CrN (10 pm), base oil. All other sliding pair systems studied fall somewhere between the pairs listed above. Each sliding pair system is discussed in general terms beiow (see Table 2 and Fig. 8).
52
Y.L. Su, J.S. Lin I Tribological potential of PVD coatings for machine elements TABLE 3. Tribological performance Sliding pairs
_
(a)
_ .-__
@I
Fig. 15. (a) The wear debris from pm) that had been paired with an wear conditions; (b) the wear debris (10 pm) that had been paired with pm) under dry wear conditions.
a TiN coated specimen (3 SKH51 specimen under dry from a CrN coated specimen a CrN coated specimen (10
4.1. Surface treated steels (CARB or TFl-SKH51)
Under dry wear conditions, CARB or TFl specimens mated with SKH51 specimens possess the worst wear resistance of all the sliding pairs. Wear resistance is remarkably improved under lubricated wear conditions, with cutting fluid yielding the best performance. 4.2. Su$ace treated steels (CARB or TFl)-PYD (CrN or TiN+ CrN)
coated
Under base oil lubrication, both surface treated steels and PVD coated parts possess very poor wear resistance, even worse than under dry wear conditions. On the other hand, water based cutting fluid lubrication is very suitable for these sliding pairs, with only polishing being exhibited in the lower specimen in most cases. Note that under both dry and lubricated wear conditions, the carburized steel and tufftrided specimens exhibit similar wear resistance. The poor wear resistance of surface treated steels paired with PVD coated specimens may be due to the higher coefficient of friction (see Fig. 9(a)) inducing higher contact stress, causing both sides of the sliding surfaces to show fatigue cracking and spalling (see Figs. 11(b), 12(a) and 13(c)); the spalled material will interfere with the lubrication and widen the scope of failure. 4.3. PKD coated specimens (TiN or CrN)-Sk7951
This sliding pair possesses poor wear resistance under dry wear; the wear scar width of specimens is much lower than that of surface treated steels paired with SKHSl. This is due to the higher hardness of PVD coated specimens. The wear resistance under cutting fluid lubrication is better than under base oil lubrication. 4.4. PKD coated specimens (TiN or CrN)-PVD
coated
(CrN or TiN+ CrN)
Although poor wear resistance is obtained under dry wear conditions, especially for CrN-TiN + CrN and CrN-CrN pairs, PVD-PVD sliding pairs possess good
ofvarious sliding pairs studied
Performance
Cutting fluid
Dry
Base oil
CARB-SKHS 1 Tufftrided-SKH51
Bad
Adequate
Excellent
TiN-SKH51 CrN-SKH5 1
Bad
Adequate
Excellent
CARB-TIN + CrN CARB-CrN Tufftrided-TiN + CrN Tufftrided-CrN
Adequate
Bad
Excellent
TiN-TiN + CrN TiN-CrN CrN-TiN + CrN CrN-CrN
Bad
Excellent
Excellent
wear resistance under cutting fluid or base oil lubrication. Poor performance under dry wear conditions is thought to arise from strong adhesion and the relatively high degree of hardness of self-mated sliding pairs. The dual TiN +CrN coating demonstrates no better wear resistance than a single layer of CrN coating. The wear performance of the four sliding pairs mentioned above is summarized in Table 3. 4.5. Suitable sliding pair for VLSTM The following sliding (upper-lower) pairs are suitable for VLSTM applications: (1) surface treated steels paired with SKHSl, TiN+CrN or CrN PVD coated specimens under cutting fluid lubrication; (2) TiN or CrN PVD coated specimens paired with SKH51 specimens under cutting fluid lubrication; (3) TiN or CrN PVD coated specimens paired with TIN + CrN or CrN PVD coated specimens under base oil and cutting fluid lubrication.
5. Conclusions (1) Sliding pairs generally possessed the best wear resistance under cutting fluid lubrication. Although surface treated steels paired with CrN or TiN+CrN PVD coated parts exhibited very poor wear resistance under base oil lubrication, even worse than under dry wear conditions, wear performance was remarkably better under cutting fluid lubrication. (2) PVD coated specimens paired with surface treated steels under dry wear conditions exhibited a transferred layer composed mostly of iron. (3) Under cutting fluid lubrication, the worn surface of PVD lower specimens consistently exhibited surface polishing (no measurable wear). These results were independent of upper pair choice.
XL. Su, J.S. Lin I Tribological potential of PVD coatings for machine elements
(4) Suitable sliding pairs for VLSTM applications include: surface treated steels paired with either SIMS1 or PVD coated specimens under cutting fluid lubrication; PVD coated specimens paired with SKH51 specimens under cutting fluid lubrication; and PVD coated specimens paired with PVD coated specimens under base oil and cutting fluid lubrication.
Acknowledgments The authors are grateful to the National Science Council of the Republic of China for supporting this research under grant NSC79-0414-EOO6-105R. This project was directed by Dr. Hong-Sen Yan and Dr. Rong-Shean Lee.
References
53
4 C.T. Young and S.K. Rhee, Wear processes of TiN-coated drills, Proc. ASMEISTLE Joint Symp. on Wear of Materials, Houston, TX, 1987, pp. 543-550. 5 W.D. Sproul, Thin Solid Films, 26 (1985) 257-263. 6 Z. Palmai, Wear, 95 (1984) l-7. 7 F.A. Soliman and O.A. Abu-Zeid, Wear, I19 (1987) 199-204. 8 H. Randhawa, J. Vat. Sci. TechnoZ., A4 (6) (1986) 2755-2758. 9 R. Buhl, H.K. Pulker and E. Moll, Thin Solid Films, 80 (1981) 265-270. 10 A.K. Chattopadhyay and A.B. Chattopadhyay, Wear, 80 (1982) 239-258. 11 M.J. Park, A. Leyland and A. Matthews, Surf: Coat. Technol., 43144 (1990) 481492. 12 E. Rabinowicz, ASLE Trans., 14 (1971) 198-205. 13 Y.L. Su, J.S. Lin, L.I. Shiau and J.D. Wu, Wear, 167 (1993) 73-83. 14 Development and Practical Application of the Tz@rided Process (Old and New). Degussa company technical information, Hanau, Germany, 1975. 15 T. Arai, H. Fujita and M. Watanabe, Thin Solid Films, 154 (1987) 387-401. between 16 S.J. Bull and D.S. Rickerby, The inter-relationship coating microstructure and the tribological performance of PVD coatings, in D. Dowson, C.M. Taylor and M. Godet (eds.), Mechanics of Coatings, Proc. 16th Leeds-Lyon Symp. on Tkbolog~, 1989, pp. 3371348. 17 0. Knotek. F. Loffler and H.J. Scholl. , Surt: Coat. Technol.. 45 (1991) 53-58. of 18 A.W. Ruff, L.K. Ives and W.A. Glaeser, Characterization wear surface and wear debris, in D.A. Rigney (ed.), Fundamentals of Friction and Wear of Materials, American Society for Metals, Metals Park, OH, 1981, pp. 235-289. ”
1 L. Pezzoli, United States Patent 4 624 288, 1986. 2 Y.L. Su and J.S. Lin, Wear, 166 (1993) 27-35. 3 Y.L. Su and J.S. Lin, Wear, 160 (1993) 139-151.