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TiN multilayer coatings
SCT-18484; No of Pages 9 Surface & Coatings Technology xxx (2013) xxx–xxx
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Cavitation erosion resistance of Ti/TiN multilayer coatings Alicja K. Krella ⁎ The Szewalski Institute of Fluid-Flow Machinery, Polish Academy of Sciences, Fiszera 14, 80-231 Gdansk, Poland
a r t i c l e
i n f o
Article history: Received 10 April 2012 Accepted in revised form 6 April 2013 Available online xxxx Keywords: Multilayer coatings Arc PVD coatings Titanium nitride–titanium Cavitation erosion Fracture
a b s t r a c t The results of cavitation erosion tests of the Ti/TiN multilayer coatings protecting the X6CrNiTi18-10 steel are presented in this paper. The Ti/TiN multilayer coatings were deposited on the X6CrNiTi18-10 steel surface by means of the cathodic arc evaporation PVD method. The multilayer coatings were deposited as 4, 12 and 40-layer structures with the Ti layer at the bottom and the TiN layer on the top. All layers showed similar thickness; the total thickness of the Ti/TiN multilayer coatings was approximately 4 μm. The coated specimens showed better cavitation erosion resistance than the uncoated stainless steel. With an increase of the number of coating layers the cavitation erosion resistance of the Ti/TiN multilayer coating was lower. It was shown experimentally that the Ti/TiN-4 coating provided the best protection against cavitation erosion. The degree of cavitation erosion was estimated by mass measurements and surface roughness measurement tests. The value of surface roughness of the Ti/TiN-4 coating was closest to that of the uncoated steel. The surface profiles of the Ti/TiN-12 and Ti/TiN-40 coatings showed little undulation, not visible in the profiles of the Ti/TiN-4 coating and the uncoated X6CrNiTi18-10 steel. The microscopic observation showed that deformation of the multilayer coatings developed via cracking, which further propagated by simultaneous fracture of numerous coating layers. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Development of protective coating technology is stimulated by the industry demand to increase the effectiveness of working machines. Advanced coatings, like multilayer ones, are of special interest due to their capability to have unique properties, unattainable for monolayer coatings. The Ti/TiN multilayer coatings belong to an interesting group of protective coatings because it combines the properties of hard TiN with those of soft Ti layers. Investigations of mechanical properties [1] have shown that along with an increase of Ti layer thickness, the plasticity of the Ti/TiN coatings increased, but hardness and elastic modulus of the multilayer coatings fell down. The soft Ti interlayer behaved like a solid lubricant during wear tests and the thinner the soft Ti interlayer the lower the friction coefficient [1]. Also, studies of abrasion and particle erosion [2] have shown the decrease of wear and erosion rates with a decrease of the thickness of Ti layer. The lowest erosion rate was achieved for the Ti/TiN multilayer coating with 0.1-μm-thick Ti layer [2]. Investigations performed by Wiklund et al. [3] have shown that the Ti/TiN multilayer coating possesses nearly three times higher cracking resistance than the TiN coating. Very high cracking resistance of the Ti/TiN coating was attributed to the branching of cracks at the interfaces together with blunting of a crack tip caused by a soft Ti layer. Also according to Ref. [4], the deposition of the TiN/Ti coating on the TC17 ⁎ Tel.: +48 58 69952355. E-mail address: [email protected].
titanium alloy substrate improved the fretting fatigue resistance of the titanium alloy. As shown in Ref. [4], the fretting fatigue endurance of the TC17 alloy covered with the TiN/Ti coating increased approximately three times in comparison with that of the TC17 alloy coated with the TiN monolayer coating. Very good fatigue resistance of the TiN/Ti coating was explained by solid lubricant function of soft Ti particles which absorb the fretting energy. Positive results of fatigue investigation [3,4], have promised a good resistance of the TiN/Ti multilayer coatings against cavitation. The aim of this paper is to evaluate cavitation erosion resistance of TiN/Ti multilayer coatings deposited on austenitic stainless steel depending on the number of layers. 2. Experimental techniques 2.1. Materials The Ti/TiN multilayer coatings were deposited on the X6CrNiTi18-10 steel substrate by cathodic arc evaporation (Arc) PVD method. The X6CrNiTi18-10 steel was subjected to heat treatment at 1050 °C and then water was quenched in order to remove residual stress, twins, martensite and most dislocations. Before the deposition the surface of substrate was polished; after polishing the surface roughness was Ra = 0.02 μm. The deposition parameters are shown in Table 1. The Ti/TiN multilayer coatings were produced as four-, twelve- and forty-layer coatings. The Ti layers were deposited alternatively with the TiN layers starting with the Ti layer. The TiN layer was always
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A.K. Krella / Surface & Coatings Technology xxx (2013) xxx–xxx Table 1 Deposition parameters of Ti/TiN multilayer coatings. 2 × 10−3 Pa 1 Pa 1 Pa 85 A −100 V 350 °C 20 s–3 min
Pressure of residual gases Working pressure of argon Working pressure of nitrogen Arc current Substrate bias voltage Substrate temperature Time of deposition of each layer
deposited on top. The thickness of the Ti layer was approximately the same as that of the TiN layer. The total multilayer coating thickness was approximately 4 μm. The coatings were marked by the symbol of coating type and the total number of layers, for example Ti/TiN-4. The phase composition of the coatings was investigated at a DRON2 X-ray diffractometer using the CuKα radiation at the Koszalin University of Technology, Poland, Institute of Mechatronics, Nanotechnology and Vacuum Technique. The lattice parameters were determined using the X-ray diffraction pattern. The Ti and TiN grain sizes were determined using the X-ray diffractometer and the Scherrer equation with Gaussian profile adjustment. The hardness and Young's modulus were measured with a NanoHardness Tester (CSEM) using the method of Oliver and Pharr. A scratch tester Revetest® made by CSEM was used to investigate the coating adhesion. 2.2. Procedure of cavitation erosion test The experimental tests were performed in a cavitation tunnel with a system of barricades. The chamber was presented in Ref. [5]. Flow conditions are defined by the p1 and p2 absolute pressures measured at the chamber inlet and outlet respectively, and by the slot width. The specimens (45 × 26 × 14 mm) were subjected to cavitation at p1 = 1000 kPa inlet pressure, p2 = 130 kPa outlet pressure and the inter-barricade slot width of 5 mm. Each cavitation test lasted 600 min and consisted of 11 successive exposures as follows: for the first 180 min of test the specimens were exposed to cavitation impingement in 30 min long intervals, afterwards three test exposures of 1 h and two exposures of 2 h were applied. Before the test and after each test interval the specimens were cleaned, dried and reweighed using an analytical balance with sensitivity of 0.1 mg. Each test was repeated twice and the average values were used for further analysis. Simultaneously with the mass loss measurements the surface roughness parameters were measured by means of the SJ-301 Mitutoyo Surface Roughness Tester. 3. Results 3.1. Microstructural characteristics The X-ray diffraction pattern shown in Fig. 1 reveals that preferred orientations of TiN is (111) and (200) planes. Moreover, titanium nitride has, in fact, TiN0.9 structure. The interplanar spacing of the (111) plane resulted from the X-ray diffraction pattern equals d111 = 2.447 Å. Thus, lattice parameter equals a = 4.239 Å. The same value of lattice parameter is obtained by calculating the interplanar spacings for the
Table 2 Phase composition and structure of Ti and TiN layers. Layer
TiN Ti
Chemical composition [at.%]
Phase composition
Structure Crystallographic orientation
Grain size [nm]
Ti-48,9 N-49,0 O-2,1 Ti-97,9 O-2,1
δ-TiN + Ti(N) α-Ti
(111) (010), (011)
~16 ~55
(200) and (220) planes. This value of lattice parameter of the TiN is similar to that showed in Ref. [6]. Calculated from the X-ray diffraction pattern the interplanar spacing for the (100) plane of titanium that has HCP structure is d100 = 2.555 Å and lattice parameter is a = 2.950 Å. The interplanar spacings calculated for the (002) and (004) planes are d002 = 2.341 Å and d004 = 1.170 Å, respectively. This allows obtaining lattice parameter c = 4.682 Å. Similar values of lattice parameters of titanium were obtained by Wood [7]. The structures of the Ti/TiN multilayer coatings are shown in Fig. 2 writing and in Table 2. Micro-droplets seen at the coating surface are a typical consequence of using the Arc PVD method. The chemical analysis revealed their composition with a different stoichiometry than that of the surrounding TiN grains. The EDS analysis has shown that the microdroplets contained 63 to 86 at.% titanium, 13 to 36 at.% nitrogen and approximately 1 at.% oxygen, therefore they were marked as Ti(N). The size of the Ti(N) micro-droplets has wide scatter; with diameter in the range between 0.5 μm and 2 μm. The height of most of the micro-droplets was approximately 1 μm, but the height of a few micro-droplets was up to 3 μm. The micro-droplet shape was close to that of a sphere or an oval. The micro-droplets are prone to severe cavitation erosion; they reduce the incubation period and accelerate the erosion [5,8]. Münsterer and Kohlhof [8] were the first, who noticed that all defects in the coatings acted as nucleation centres for wear reducing the incubation time. In Ref. [5] it was noticed that microcracks were initiated at the craters arisen from the removed Ti(N) micro-droplets and at the boundary between the micro-droplets and the surrounding TiN coating. It was also noticed in this paper, that the large number of very small microdroplets did not determine the site of crack initiation but cracks were initiated at spots of microjet impacts. In the present investigations with increasing number of layers, the number of micro-droplets has also increased, but their size remained approximately unchanged that is reflected in the Ra value measured before the cavitation test (Table 3).
3.2. Cavitation erosion Cavitation erosion curves of the Ti/TiN multilayer coatings and the uncoated X6CrNiTi18-10 stainless steel are shown in Fig. 3. The shape of erosion curves of the Ti/iN multilayer coatings differs significantly from that of the X6CrNiTi18-10 steel. Mass losses of the Ti/TiN coatings were detected after a shorter exposure than in the case of the stainless steel. Incubation period of the uncoated steel was 90 min and it was the longest among all tested specimens. The incubation period of the Ti/TiN-4 coating was 60 min, while in the case of the Ti/TiN-12 and Ti/TiN-40 coatings the incubation periods were shorter than 30 min. At the beginning of cavitation tests (the initial 150 min of exposure), the mass losses of coated specimens were bigger than those of the uncoated specimen. This effect is especially seen in case of the Ti/TiN-40 coating. With the cavitation test duration increase, the mass loss rate of the coated specimens decreased. After the whole cavitation test, the cumulative mass losses of coated specimens were much lower than those of the uncoated specimen. The mass loss rates of the coated specimens were related to the surface roughness and the number of the Ti(N) microdroplets. The microscopic observations suggest that degradation of the coated specimens started by the removal of microdroplets from the surface of the coatings. The mass loss of the Ti/TiN-40 coating, showing the largest amount of microdroplets (the highest Ra parameter), was the highest. Lack of the mass loss of the Ti/TiN-4 coating during the first 60 min of exposure could be due to the lowest number of microdroplets and higher thickness of each layer in this multilayer coating in comparison to the thickness of layers in other multilayer coatings.
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A.K. Krella / Surface & Coatings Technology xxx (2013) xxx–xxx
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Fig. 1. XRD pattern of the Ti/TiN-4 coating.
3.3. Roughness changes Measurements of surface roughness of all tested specimens were performed simultaneously with the mass measurements. The changes of the surface roughness parameter, Ra, with the test duration are shown in Fig. 4. During the first 150 min of the cavitation test the surface roughness of the uncoated stainless steel increased from Ra = 0.02 μm to 1.01 μm and during the rest of cavitation test — by another 1 μm, up to Ra = 1.95 μm. In case of the Ti/TiN-4, Ti/TiN-12 and Ti/TiN-40 coatings, the surfaces roughness increased during the first 150 min of exposure from Ra = 0.3, 0.34 and 0.36 μm up to 0.74, 0.7 and 0.71 μm, respectively. The highest increase of surface roughness parameter in the case of the Ti/TiN-4 coating and the lowest one in the case of the Ti/TiN-40 coating are very symptomatic. During the rest of cavitation tests the surface roughness of the Ti/TiN-4, Ti/TiN-12 and Ti/TiN-40 coatings increased up to Ra = 1.75, 1.29 and 1.26 μm, respectively. These results show that the highest increase in surface roughness of the Ti/TiN-4 coating, close to that of the uncoated steel, is a general trend. The surface roughness of the Ti/TiN-12 coating increased in a similar rate to that of the Ti/TiN-40 coating. Comparing the mass loss curves (Fig. 3) with changes in surface roughness (Fig. 4), some similarity can be noticed. The system with Ti/TiN-4 coating has the lowest mass loss among all tested multilayer coatings and its roughness (Ra parameter) is the closest to that of the uncoated steel. The system with Ti/TiN-40 coating has the highest mass loss among all tested multilayer coatings and the changes in its roughness were the lowest of all. Performed measurements prove that an increase in Ra parameter of the employed systems accompanies a decrease in the mass losses of the systems with multilayer coatings. A similar result was reported in Ref. [5]. The surface profiles of all tested specimens after cavitation tests are presented in Fig. 5. The total height of the profile (the sum of the maximum profile peak height and the maximum profile valley depth over the evaluation length) of the Ti/TiN-40 coating is comparable to that of the Ti/TiN-12 coating and much lower than that of the Ti/TiN-4 coating and the uncoated X6CrNiTi18-10 steel. Moreover, the surface profiles of the Ti/TiN-12 and Ti/TiN-40 coatings show smaller variations that cannot be noticed in the profiles of the Ti/TiN-4 coating and those of the uncoated X6CrNiTi18-10 steel. The mean depth and width of the Ti/TiN-12 and Ti/TiN-40 coating surface
valleys were very close to each other, and equal approximately 3 μm and 150 μm, respectively. The surface roughness profiles of the Ti/TiN-12 and Ti/TiN-40 coatings suggest the similarity in deformation/ degradation process. In the case of the Ti/TiN-4 coating, the mean value of the depth and of the width of surface valleys was 7 μm and 250 μm, respectively. Taking into account that the depth and the width of the valleys of the uncoated steel were 8 μm and 300 μm, respectively, the surface of the Ti/TiN-4 coating clearly resembled that of the uncoated steel. The rough microscopic observations (Fig. 6) confirmed the difference in the deformation between the Ti/TiN multilayer coatings. All the multilayer coatings deformed causing micro-folding. The undulation of the Ti/TiN-4 multilayer coating was most evident. The degree of irregularities decreased with the rising number of layers, which was also obtained using the surface roughness tester. 3.4. Fracture Higher magnification observation has shown that the Ti/TiN multilayer coatings deformed via cracking. First cracks occurred on the top of the micro-folding (Fig. 7). Fig. 7 shows that cracks propagated via simultaneous fracture of many coating layers. Some Ti(N) microdroplets were removed from the surface of the coating, others remained at their original sites. The removed Ti(N) micro-droplets left pits. However, some pits were caused by cavitation microjet penetration. It is difficult to distinguish pits created by the micro-droplet removal and those created by microjets. 4. Discussion 4.1. Properties of coatings The coatings were deposited on the surface of substrate by a condensation process, so the coating bonding to the substrate showed a typical adhesive nature. Because the deposition process proceeded at approximately 350 °C, the thermal properties of the substrate, the Ti coating layer and the TiN layer could influence the thermal mismatch, thermal stresses and the coating adhesion. The difference in thermal expansion between TiN (9.35 μm/m °C) and Ti (8.9 μm/m °C) is low, hence the thermal mismatch and thermal stresses are probably also low. The thermal expansion of the Ti(N) microdroplets is assumed to
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A.K. Krella / Surface & Coatings Technology xxx (2013) xxx–xxx
a1
a2
b1
b2
c1
c2
Fig. 2. Cross-sectional micrographs of TiN/Ti multilayer coatings.
be between those of the TiN and the Ti. Thus, the thermal mismatch between the TiN or the Ti layer and the Ti(N) microdroplets should be lower than that between the Ti and the TiN layer. The thermal
expansion coefficient of the substrate (the X6CrNiTi18-10 steel) is 18.70 μm/m °C, which is approximately twice as high as that of Ti. This high difference suggests the risk of high thermal stresses between
Table 3 Properties of Ti/TiN multilayer coatings. Coating
Ti/TiN-4 Ti/TiN-12 Ti/TiN-40
Thickness of coating [μm]
3.7 3.7 3.6
Number of layers
4 12 40
Thickness of single layer [μm]
0.925 0.31 0.09
Hardness of coating [GPa]
Young's modulus of coating [GPa]
Adhesion LC2 [N]
Roughness Ra [μm]
Mean value
Standard deviation
Mean value
Standard deviation
Mean value
Standard deviation
Mean value
Standard deviation
17 16 11
3.14 3.33 3.99
307 348 286
56 108 133
19 19 15
0.87 1.15 1.56
0.3 0.34 0.36
0.02 0.05 0.038
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A.K. Krella / Surface & Coatings Technology xxx (2013) xxx–xxx
7
5
2 X6CrNiTi18-10
5
Ti/TiN-12
1.6
Ti/TiN-40
1.4
4
Ra, um
mass loss, mg
1.8
Ti/TiN-4
6
3
1.2 1 0.8 1H18N9T
0.6
2
Ti/TiN-4
0.4 1
Ti/TiN-12
0.2 0
Ti/TiN-40
0 0
100
200
300
400
500
600
0
100
200
300
400
500
600
time, min
time, min Fig. 3. Cavitation curves.
Fig. 4. Changes of surface roughness with test duration.
the Ti layer and the substrate. Taking into account the risk of thermal mismatch between the multilayer coating and the substrate, low hardness of the X6CrNiTi18-10 steel and its large plastic deformation under substantial pressure of the Rockwell indenter used for the scratch test, the adhesion of the multilayer coatings to the substrate can be considered to be relatively good (Table 3). Because Ti shows the HCP structure with lattice parameters a = 2.95 Å, c = 4.68 Å and TiN shows the FCC structure with lattice parameter a = 4.24 Å, there is a possibility of atomic chemisorptions at the interface between the Ti layer and TiN layer. Moreover, according to Ref. [9], every new layer in a multilayer coating tends to grow in a close structural relationship with the previous layer. Considering low thermal mismatch, the possibility of developing a close structural correlation between the Ti and TiN layers and the possibility of atomic chemisorption between Ti and TiN layers, the adhesion between these two layers should be high. Lugscheider et al. [10] have shown that the microstructure of PVD films, including Arc PVD coatings, changes from a columnar form to a dense and uniform structure with the increase of deposition temperature. TEM investigations of Cr/CrN multilayer coating structure as deposited by the Arc PVD method at 380–420 °C [11] showed that all layers in the multilayer coating had a dense columnar nanocrystalline structure. Also investigations performed by Suresha et al. [12] showed that TiN/(AlTi)N multilayer coatings obtained by the Arc PVD method show columnar microstructure. Moreover, Ref. [12] reports on the columnar growth undisrupted by the TiN/(AlTi)N interface with the columnar grains extending through the entire thickness of the coating. This is in agreement with the Holleck's and Schier's statement [9]. Because titanium nitride and titanium layers were deposited using the Arc PVD method with deposition temperature of 350 °C, so they had columnar nanocrystalline grain structures extending throughout the layer thickness and exhibit anisotropy in elastic and fracture behaviour. Holleck and Schier [9] noticed that an increase in the number of interfaces in the fine grained structures of multilayer PVD coatings often contributes to an increase in crack propagation resistance and toughness due to energy dissipation and stress relaxation at the interfaces. With the increase of number of layers in the employed Ti/TiN multilayer coatings, the number of interfaces increases as well. Moreover, the increase of the number of microdroplets appearing at the coating surface along with the increase of the number of layers in the Ti/TiN multilayer coatings contribute also to the growth of interfaces. Thus, according to Ref. [9], the Ti/TiN-40 multilayer coating should have much better crack propagation resistance than the Ti/TiN-4 one. The performed investigation shows the opposite
relationship, which is probably related to an increase in the number of micro-droplets with the number of layers increasing. Performed investigation of the cavitation erosion resistance shows that more and more microdroplets are removed, leaving empty pinholes, in the beginning of a cavitation test when the number of layers (which is equivalent to the decrease of each layer thickness — Table 3) in the Ti/TiN multilayer coatings increases. It is logical that the low-embedded micro-droplets are removed as the first ones. This investigation shows that the decrease of thickness of each layer in a multilayer coating causes an increase in the number of low-embedded Ti(N) microdroplets. After removing the low-embedded Ti(N) microdroplets the erosion rate falls down. 4.2. Hardness The ability of examined material to deform depends on mechanical properties and its structure. The hardness of Ti/TiN multilayer coatings depends on the ratio of soft titanium to hard titanium nitride contents. Although the amount of soft titanium remained approximately unchanged with the increasing numbers of layers in the employed Ti/TiN multilayer coating, the hardness of the coating decreased (Table 3). Fig. 8 shows the relationship between the thickness of the layers and the hardness of the Ti/TiN multilayer coating. The decrease of layer thickness from 310 nm to 90 nm caused rapid decrease of hardness of the whole Ti/TiN multilayer coating. Santana et al. [13] have used a simple rule of mixture to estimate the hardness of multilayer coating. They have discovered that the calculated hardness of multilayer coating was lower than the measured one. They have concluded that the enhancement of hardness was caused by the interface that had influence on crack propagation. Similar results were obtained by Chen et al. [14]. Applying the rule of mixture to the deposited Ti/TiN multilayer coatings, Santana et al. [13] have determined the following formula (equation for the hardness of the composite coating): Hcomposite ¼
tTi t H þ TiN H ttotal Ti ttotal TiN
ð1Þ
where t is the thickness of each layer and H is its hardness. Taking into account that the thickness of all Ti layers and all TiN layers remained nearly the same, the calculated multilayer hardness should be unchanged for all multilayer coatings despite the increasing number of layers. The average hardness of the coating deposited in our experiments, determined from Eq. (1) is 10.9 GPa. Assuming that the interface enhances the hardness of multilayer coating, as it
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15.0 10.0
a)
X6CrNiTI18-10
[um]
5.0 0.0 -5.0 -10.0 -15.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
[mm] 15.0 10.0
b)
Ti/TiN-4
[um]
5.0 0.0 -5.0 -10.0 -15.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
[mm] 15.0 10.0
c)
Ti/TiN-12
[um]
5.0 0.0 -5.0 -10.0 -15.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
[mm] 15.0
d)
Ti/TiN-40
10.0
[um]
5.0 0.0 -5.0 -10.0 -15.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
[mm] Fig. 5. Roughness profiles of tested specimens after cavitation test; a) the profile of the X6CrNiTi18-10 steel, b) the profile of the Ti/TiN-4 multilayer coating, c) the profile of the Ti/TiN-12 multilayer coating, and d) the profile of the Ti/TiN-40 multilayer coating.
was proved in Ref. [13,14], the hardness of the coatings should increase with increasing number of layers. However, the measurements of hardness in our experiments have shown an opposite rule. The hardness of the Ti/TiN-40 multilayer coating was the smallest and close to the calculated value, and the hardness of the Ti/TiN-4 multilayer coating was the highest. It can be supposed that micro-droplets might be responsible for the decreasing hardness of the multilayer coatings. Performed investigations showed that with increasing number of layers, the number of micro-droplets per unit surface also increases. Nevertheless, an increase of the number of micro-droplets was not as clear as the drop of hardness. With increasing roughness of the coating from
0.30 μm for the Ti/TiN-4 coating to 0.34 μm for the Ti/TiN-12 coating and to 0.36 μm for the Ti/TiN-40 coating, the hardness dropped from 17 GPa for the Ti/TiN-4 coating to 16 GPa for the Ti/TiN-12 coating and to 11 GPa for the Ti/TiN-40 coating. In the case of the Ti/TiN-12 coating, the number of interlayers increased from 4 to 12 and the roughness increased from 0.30 to 0.34 μm, while, as a result, the hardness decreased by 1 GPa. In case of the Ti/TiN-40 coating, the number of interlayers was 40, but compared to Ti/TiN-12 coating the roughnesses differ only slightly from 0.34 μm (Ti/TiN-12) to 0.36 μm. Although following the results of Santana et al. [13] and Chen et al. [14] it could be expected that the hardness should not change significantly, in fact,
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A.K. Krella / Surface & Coatings Technology xxx (2013) xxx–xxx
Ti / TiN-4
7
a)
b) Ti / TiN-12
c) Ti / TiN-40
Fig. 7. Surface profiles of degraded specimen after cavitation test; a) Ti/TiN-4 coating, b) Ti/TiN-12 coating, and c) Ti/TiN-40 coating.
Fig. 6. Surface profiles of tested specimens after cavitation test.
it decreased rapidly from 17 GPa to 11 GPa. This result shows that the influence of the interfaces on the multilayer coating hardness is not as simple as it was proved by Santana et al. [13] and Chen et al. [14]. In present investigations TiN, whose hardness is higher than that of Ti, was always the outer layer. The indentation depth during the measurements of coating hardness should not exceed 1/10 of the coating thickness. In present measurements the maximum indentation depth was 200 nm. The thickness of the TiN top layer in the Ti/TiN-4 coating was approximately 920 nm. Taking into account the indentation depth and the thickness of top layer, the conclusion can be that the hardness of the top layer dominated the hardness of the whole multilayer coating. In case of the Ti/TiN-12 coating, the thickness of the top TiN layer was approximately 300 nm. So, the
indentation depth was nearly the same as the thickness of the top layer. Thus, the soft Ti layer just under the TiN layer contributed significantly to the total hardness. In case of the Ti/TiN-40 coating, the thickness of the top TiN layer was approximately 90 nm. The indenter during hardness measurements penetrated through the whole thickness of the TiN and Ti layer. Thus, the influence of the soft Ti layer on the total hardness increased. Probably, therefore, the hardness of this coating was close to the calculated hardness. Performed investigations suggest that the hardness and thickness of outer layer have significant influence on the hardness of the whole multilayer coating. 4.3. Fracture Performed cavitation erosion tests have shown good protective properties of all tested Ti/TiN multilayer coatings (Fig. 3). Nevertheless, the protective properties lessened with the decrease of the layer thickness (decrease of the coating hardness) (Fig. 9). Correlation between
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Fig. 8. Correlation between coating hardness and thickness of layers in TiN/Ti coatings.
the mass loss and the coating hardness was investigated also in Ref. [5], where the influence of mechanical properties of TiN monolayer coatings, as deposited at different technological parameters, on cavitation erosion resistance was investigated. Ref. [5] shows that for the TiN monolayer coating there exists a critical value of hardness. An increase of hardness in the subcritical domain results in a decrease of the mass loss whereas mass loss increase is accompanied by the rise of the coating hardness for the supercritical hardness values. This was explained by the absorption of energy being used for the deformation of the TiN monolayer coating, crack generation and their propagation [5]. In this study, the increase of the mass loss with the decrease of hardness of the multilayer coating can be also related to the absorption of energy on deformation and crack initiation, especially by the Ti layers, the Ti(N) microdroplets and the interfaces. To analyse the process of material degradation by cavitation, it is important to know the factors that cause the damage. Cavitation erosion is caused by repeated action of cavitation bubbles collapsing in the vicinity of the solid surface. The high speed camera observations of the cavitation bubbles collapse [15–17] which have revealed that the bubbles lose their spherical shape, and emit shock waves and microjets. Measurements of material loading caused by cavitation bubbles [16–20] show a wide range of amplitude from tens of kPa up to hundreds of MPa. Generally, the microjets are directed towards the solid surface [15–17], such direction of microjet has been assumed also in the present analysis.
Fig. 9. Correlation between mass loss and hardness of TiN/Ti coatings.
Due to the wide range of microjet speed and amplitude of the impacting shock waves [15–22], the solid material is passing locally from elastic deformation to adiabatic shearing. Such loading may activate many deformation mechanisms. Due to assumed columnar structure of the Ti/TiN multilayer coating, shear cracking and sliding along columnar grain boundaries are the easiest deformation mechanisms for such materials. Due to the TiN structure resembles that of the NaCl, dislocations show high activation energy. On the other hand, activation energy decreases in nanocrystalline ceramics [23]. Investigations presented in Ref. [23,24] show that during the deposition process dislocations were generated in the TiN grains and the external stress was able to induce dislocation motion at room temperature. For this reason it is assumed that dislocations occur in TiN layer, deposited by Arc PVD, and deformation mechanisms such as dislocation motion, sliding/shearing along grain boundaries, and shear band formation can be activated by cavitation pulses. In the case of Ti grains, similar deformation mechanisms are activated, but because the size of Ti grains is approximately three times bigger than that of the TiN grains, and due to low dislocation activation energy, the contribution of dislocation motion to the whole deformation is expected to be larger than that in the TiN grains. The microjet impacts the surface of the coating with high impact speed. The coating is compressed down in a single direction. The generated stresses propagate within the coating as a wave, but each activated mechanism needs some energy, so finally the stresses and waves disappear. In general, the dislocations are forced to motion if stress exceeds some critical level, which increases with rising strain rate and decreases with rising temperature. In the TiN grains the stress needed to move a dislocation, σyTiN, is high due to the ionic bonding, while stress needed to cause sliding/shearing along grain boundary, σgbTiN, is much lower than σyTiN due to columnar structure of the grains. In the same way, the stress needed to move dislocation in Ti grains, σyTi, is higher than the stress needed to cause sliding/ shearing along grain boundary, σgbTi. Moreover, the stresses σyTiN and σgbTiN are higher than σyTi and σgbTi, respectively, due to the TiN and Ti bonding nature. When external stress, σ, is applied to the TiN grains, it runs mainly through the grain boundary, because σgbTiN ≪ σyTiN. If the impact energy is high enough, then not only sliding but also fracture along grain boundary occurs. Crack will propagate throughout the TiN layer. On the other hand, the investigation of Kumar et al. [23,24] has indicated that the dislocation motion in the TiN grains under external stress is possible. Because of high σyTiN, some stresses that run in the plane perpendicular to the impact plane release activating dislocations — some of them are blocked by obstacles, e.g. anchored dislocation, increasing the stress level in the TiN grains. To summarize, some of impact stresses cause dislocation motion inside the grains, but most of them cause sliding and fracture along grain boundaries. Because of the columnar structure of the grains, sliding along grain boundary is the dominating deformation mechanism in the Ti layer. Due to the size of the grains and the rate of impact, the shear bands may be created. The dislocation motion is also expected to occur. Assuming that stress concentration at the crack tip is bigger than σyTi, a plastically deformed zone is generated in grains as a result of the dislocation motion. On the other hand, due to high stress wave velocity, the shear bands may be initiated inside the Ti grains. Consequently, some impact energy is absorbed and the level of the shock wave that induced stresses decreases. If the remaining impact energy is high enough, the crack continues to propagate through the Ti layer to the Ti–TiN interface, where it may be halted temporarily. Besides this, the deformed Ti grains compress the underneath TiN grains. Under this pressure, if the exerted stress σ is higher than σgbTiN, sliding along grain boundary in the TiN grains can proceed. If the impact energy is still high enough, fracture/crack along grain boundary is initiated. Energy absorbed in each layer by means of all deformation mechanisms is related to the layer mechanical properties and thickness.
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A.K. Krella / Surface & Coatings Technology xxx (2013) xxx–xxx
The decrease of layer thickness from 0.925 μm to 90 nm results in a drop of energy absorption. As a result, more energy from impact of each microjet will reach the deeper layers causing their fracture. Therefore, fractures observed at the Ti/TiN-40 and Ti/TiN-12 coatings (Figs. 7 and 8) were several layers deep. The character of bonding of the Ti(N) microdroplet to the Ti layer is different from that to the TiN layer. As a result, the bonding of a Ti(N) microdroplet to the coating becomes weakened along with the decrease of the thickness of each layer. Under attack of cavitation microjets, the Ti(N) microdroplets are easily removed when the thickness of the layers decreases because of weakening their bonding to the coating and increasing microjet action depth. Thus, with the decrease of layer thickness more microdroplets are removed, and their removal occurs sooner. In the present experiments it was observed that the mass loss of the Ti/TiN-40 coating under cavitation test conditions was initiated without any incubation period and the total mass loss after the whole test was higher than that of the Ti/TiN-4 coating (Fig. 3). Another effect of the layer thickness is a possibility of deformation of the Ti/TiN-40 coating generated by impacts of low-energy cavitation microjets. Undulation of surface profiles of the Ti/TiN-40 and the Ti/TiN-12 coatings as visible in Fig. 5, was probably caused by the impacts of cavitation microjets of lower energy, which were not able to deform the thicker Ti/TiN-4 coating layers. The Ti/TiN-4 coating was deformed by impacts of microjets of higher energy. Therefore, the roughness of the surface on the Ti/TiN-4 coating was higher than that on the Ti/TiN-12 and Ti/TiN-40 coating surface. 5. Conclusions This study presents the results of cavitation tests of Ti/TiN multilayer coatings deposited on the X6CrNiTi18-10 austenitic steel carried out by means of the ARC PVD method. The obtained results and their analysis led to the following conclusions: • Deposition of Ti/TiN multilayer coatings improves cavitation erosion resistance of the protected surface. • An increase in the number of Ti(N) droplets within the coating increasing the coating roughness causes an increase of the mass loss rate at the beginning of a cavitation. • The mass loss of the Ti/TiN multilayer coatings decreases with an increase of the coating hardness and an increase of thickness of the layers. • Hardness and thickness of top layer have influence on the hardness of the multilayer coating. • All multilayer coatings underwent micro-folding at the early stage of cavitation erosion. The deformation developed via cracking, which
9
propagated by simultaneous fracture of many coating layers. The first micro-cracks appeared at the top of undulation. • Energy absorbed in each layer due to all deformation mechanisms depends on the grain structure, bonding nature, mechanical properties and thickness of the layer.
Acknowledgements The author thanks warmly Dr. Andrzej Czyzniewski, Koszalin University of Technology, Institute of Mechatronics, Nanotechnology and Vacuum Technique, for deposing the Ti/TiN multilayer coatings, and performing cross-sectional micrographs and adhesion measurements. The author thanks warmly Prof. Maria Gazda, Gdansk University of Technology, Department of Solid State Physics, for performing the X-Ray diffraction pattern of the Ti/TiN-4 coating. References [1] Y.H. Cheng, T. Browne, B. Heckerman, C. Bowman, V. Gorokhovsky, E.I. Meletis, Surf. Coat. Technol. 205 (2010) 146. [2] M. Bromark, M. Larsson, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol. 90 (1997) 217. [3] U. Wiklund, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol. 97 (1997) 773. [4] X.H. Zhang, D.X. Liu, H.B. Tan, X.F. Wang, Surf. Coat. Technol. 203 (2009) 2315. [5] A. Krella, Surf. Coat. Technol. 204 (2009) 263. [6] A. Vojvodic, C. Ruberto, B.I. Lundqvist, Surf. Sci. 600 (2006) 3619. [7] R.M. Wood, Proc. Phys. Soc. 80 (1962) 783. [8] S. Münsterer, K. Kohlhof, Surf. Coat. Technol. 74–75 (1995) 642. [9] H. Holleck, V. Schier, Surf. Coat. Technol. 76–77 (1995) 328. [10] E. Lugscheider, C. Barimani, C. Wolff, S. Guerreiro, G. Doepper, Surf. Coat. Technol. 86–87 (1996) 177. [11] P. Wieciński, J. Smolik, H. Garbacz, K.J. Kurzydłowski, Thin Solid Films 519 (2011) 4069. [12] S.J. Suresha, R. Bhide, V. Jayaram, S.K. Biswas, Mater. Sci. Eng., A 429 (2006) 252. [13] A.E. Santana, A. Karimi, V.H. Derflinger, A. Schutze, Surf. Coat. Technol. 177–178 (2004) 334. [14] L. Chen, S.Q. Wang, S.Z. Zhou, J. Li, Y.Z. Zhang, Int. J. Refract. Met. Hard Mater. 26 (2008) 456. [15] A. Philipp, W. Lauterborn, J. Fluid Mech. 361 (1998) 75. [16] I.R. Jones, D.H. Edwards, J. Fluid Mech. 7 (1960) 596. [17] Ch.E. Brennen, Oxford Engineering Science Series, 44, Oxford University Press, 1995, p. 73, (80). [18] A. Krella, A. Czyżniewski, Wear 260 (2006) 1324. [19] T. Okada, Y. Iwai, S. Hattori, N. Tanimura, Wear 184 (1995) 231. [20] S. Hattori, T. Hirose, K. Sugiyama, Wear 269 (2010) 507. [21] M.S. Plesset, A. Prosperetti, J. Fluid Mech. 9 (1977) 145. [22] N.K. Bourne, Shock Waves 11 (2002) 447. [23] S. Kumar, D.E. Wolfe, M.A. Haque, Int. J. Plast. 27 (2011) 739. [24] S. Kumar, D. Zhuo, D.E. Wolfe, J.A. Eades, M.A. Haque, Scr. Mater. 63 (2010) 196.
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Cavitation erosion resistance of Ti/TiN multilayer coatings Alicja K. Krella ⁎ The Szewalski Institute of Fluid-Flow Machinery, Polish Academy of Sciences, Fiszera 14, 80-231 Gdansk, Poland
a r t i c l e
i n f o
Article history: Received 10 April 2012 Accepted in revised form 6 April 2013 Available online xxxx Keywords: Multilayer coatings Arc PVD coatings Titanium nitride–titanium Cavitation erosion Fracture
a b s t r a c t The results of cavitation erosion tests of the Ti/TiN multilayer coatings protecting the X6CrNiTi18-10 steel are presented in this paper. The Ti/TiN multilayer coatings were deposited on the X6CrNiTi18-10 steel surface by means of the cathodic arc evaporation PVD method. The multilayer coatings were deposited as 4, 12 and 40-layer structures with the Ti layer at the bottom and the TiN layer on the top. All layers showed similar thickness; the total thickness of the Ti/TiN multilayer coatings was approximately 4 μm. The coated specimens showed better cavitation erosion resistance than the uncoated stainless steel. With an increase of the number of coating layers the cavitation erosion resistance of the Ti/TiN multilayer coating was lower. It was shown experimentally that the Ti/TiN-4 coating provided the best protection against cavitation erosion. The degree of cavitation erosion was estimated by mass measurements and surface roughness measurement tests. The value of surface roughness of the Ti/TiN-4 coating was closest to that of the uncoated steel. The surface profiles of the Ti/TiN-12 and Ti/TiN-40 coatings showed little undulation, not visible in the profiles of the Ti/TiN-4 coating and the uncoated X6CrNiTi18-10 steel. The microscopic observation showed that deformation of the multilayer coatings developed via cracking, which further propagated by simultaneous fracture of numerous coating layers. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Development of protective coating technology is stimulated by the industry demand to increase the effectiveness of working machines. Advanced coatings, like multilayer ones, are of special interest due to their capability to have unique properties, unattainable for monolayer coatings. The Ti/TiN multilayer coatings belong to an interesting group of protective coatings because it combines the properties of hard TiN with those of soft Ti layers. Investigations of mechanical properties [1] have shown that along with an increase of Ti layer thickness, the plasticity of the Ti/TiN coatings increased, but hardness and elastic modulus of the multilayer coatings fell down. The soft Ti interlayer behaved like a solid lubricant during wear tests and the thinner the soft Ti interlayer the lower the friction coefficient [1]. Also, studies of abrasion and particle erosion [2] have shown the decrease of wear and erosion rates with a decrease of the thickness of Ti layer. The lowest erosion rate was achieved for the Ti/TiN multilayer coating with 0.1-μm-thick Ti layer [2]. Investigations performed by Wiklund et al. [3] have shown that the Ti/TiN multilayer coating possesses nearly three times higher cracking resistance than the TiN coating. Very high cracking resistance of the Ti/TiN coating was attributed to the branching of cracks at the interfaces together with blunting of a crack tip caused by a soft Ti layer. Also according to Ref. [4], the deposition of the TiN/Ti coating on the TC17 ⁎ Tel.: +48 58 69952355. E-mail address: [email protected].
titanium alloy substrate improved the fretting fatigue resistance of the titanium alloy. As shown in Ref. [4], the fretting fatigue endurance of the TC17 alloy covered with the TiN/Ti coating increased approximately three times in comparison with that of the TC17 alloy coated with the TiN monolayer coating. Very good fatigue resistance of the TiN/Ti coating was explained by solid lubricant function of soft Ti particles which absorb the fretting energy. Positive results of fatigue investigation [3,4], have promised a good resistance of the TiN/Ti multilayer coatings against cavitation. The aim of this paper is to evaluate cavitation erosion resistance of TiN/Ti multilayer coatings deposited on austenitic stainless steel depending on the number of layers. 2. Experimental techniques 2.1. Materials The Ti/TiN multilayer coatings were deposited on the X6CrNiTi18-10 steel substrate by cathodic arc evaporation (Arc) PVD method. The X6CrNiTi18-10 steel was subjected to heat treatment at 1050 °C and then water was quenched in order to remove residual stress, twins, martensite and most dislocations. Before the deposition the surface of substrate was polished; after polishing the surface roughness was Ra = 0.02 μm. The deposition parameters are shown in Table 1. The Ti/TiN multilayer coatings were produced as four-, twelve- and forty-layer coatings. The Ti layers were deposited alternatively with the TiN layers starting with the Ti layer. The TiN layer was always
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A.K. Krella / Surface & Coatings Technology xxx (2013) xxx–xxx Table 1 Deposition parameters of Ti/TiN multilayer coatings. 2 × 10−3 Pa 1 Pa 1 Pa 85 A −100 V 350 °C 20 s–3 min
Pressure of residual gases Working pressure of argon Working pressure of nitrogen Arc current Substrate bias voltage Substrate temperature Time of deposition of each layer
deposited on top. The thickness of the Ti layer was approximately the same as that of the TiN layer. The total multilayer coating thickness was approximately 4 μm. The coatings were marked by the symbol of coating type and the total number of layers, for example Ti/TiN-4. The phase composition of the coatings was investigated at a DRON2 X-ray diffractometer using the CuKα radiation at the Koszalin University of Technology, Poland, Institute of Mechatronics, Nanotechnology and Vacuum Technique. The lattice parameters were determined using the X-ray diffraction pattern. The Ti and TiN grain sizes were determined using the X-ray diffractometer and the Scherrer equation with Gaussian profile adjustment. The hardness and Young's modulus were measured with a NanoHardness Tester (CSEM) using the method of Oliver and Pharr. A scratch tester Revetest® made by CSEM was used to investigate the coating adhesion. 2.2. Procedure of cavitation erosion test The experimental tests were performed in a cavitation tunnel with a system of barricades. The chamber was presented in Ref. [5]. Flow conditions are defined by the p1 and p2 absolute pressures measured at the chamber inlet and outlet respectively, and by the slot width. The specimens (45 × 26 × 14 mm) were subjected to cavitation at p1 = 1000 kPa inlet pressure, p2 = 130 kPa outlet pressure and the inter-barricade slot width of 5 mm. Each cavitation test lasted 600 min and consisted of 11 successive exposures as follows: for the first 180 min of test the specimens were exposed to cavitation impingement in 30 min long intervals, afterwards three test exposures of 1 h and two exposures of 2 h were applied. Before the test and after each test interval the specimens were cleaned, dried and reweighed using an analytical balance with sensitivity of 0.1 mg. Each test was repeated twice and the average values were used for further analysis. Simultaneously with the mass loss measurements the surface roughness parameters were measured by means of the SJ-301 Mitutoyo Surface Roughness Tester. 3. Results 3.1. Microstructural characteristics The X-ray diffraction pattern shown in Fig. 1 reveals that preferred orientations of TiN is (111) and (200) planes. Moreover, titanium nitride has, in fact, TiN0.9 structure. The interplanar spacing of the (111) plane resulted from the X-ray diffraction pattern equals d111 = 2.447 Å. Thus, lattice parameter equals a = 4.239 Å. The same value of lattice parameter is obtained by calculating the interplanar spacings for the
Table 2 Phase composition and structure of Ti and TiN layers. Layer
TiN Ti
Chemical composition [at.%]
Phase composition
Structure Crystallographic orientation
Grain size [nm]
Ti-48,9 N-49,0 O-2,1 Ti-97,9 O-2,1
δ-TiN + Ti(N) α-Ti
(111) (010), (011)
~16 ~55
(200) and (220) planes. This value of lattice parameter of the TiN is similar to that showed in Ref. [6]. Calculated from the X-ray diffraction pattern the interplanar spacing for the (100) plane of titanium that has HCP structure is d100 = 2.555 Å and lattice parameter is a = 2.950 Å. The interplanar spacings calculated for the (002) and (004) planes are d002 = 2.341 Å and d004 = 1.170 Å, respectively. This allows obtaining lattice parameter c = 4.682 Å. Similar values of lattice parameters of titanium were obtained by Wood [7]. The structures of the Ti/TiN multilayer coatings are shown in Fig. 2 writing and in Table 2. Micro-droplets seen at the coating surface are a typical consequence of using the Arc PVD method. The chemical analysis revealed their composition with a different stoichiometry than that of the surrounding TiN grains. The EDS analysis has shown that the microdroplets contained 63 to 86 at.% titanium, 13 to 36 at.% nitrogen and approximately 1 at.% oxygen, therefore they were marked as Ti(N). The size of the Ti(N) micro-droplets has wide scatter; with diameter in the range between 0.5 μm and 2 μm. The height of most of the micro-droplets was approximately 1 μm, but the height of a few micro-droplets was up to 3 μm. The micro-droplet shape was close to that of a sphere or an oval. The micro-droplets are prone to severe cavitation erosion; they reduce the incubation period and accelerate the erosion [5,8]. Münsterer and Kohlhof [8] were the first, who noticed that all defects in the coatings acted as nucleation centres for wear reducing the incubation time. In Ref. [5] it was noticed that microcracks were initiated at the craters arisen from the removed Ti(N) micro-droplets and at the boundary between the micro-droplets and the surrounding TiN coating. It was also noticed in this paper, that the large number of very small microdroplets did not determine the site of crack initiation but cracks were initiated at spots of microjet impacts. In the present investigations with increasing number of layers, the number of micro-droplets has also increased, but their size remained approximately unchanged that is reflected in the Ra value measured before the cavitation test (Table 3).
3.2. Cavitation erosion Cavitation erosion curves of the Ti/TiN multilayer coatings and the uncoated X6CrNiTi18-10 stainless steel are shown in Fig. 3. The shape of erosion curves of the Ti/iN multilayer coatings differs significantly from that of the X6CrNiTi18-10 steel. Mass losses of the Ti/TiN coatings were detected after a shorter exposure than in the case of the stainless steel. Incubation period of the uncoated steel was 90 min and it was the longest among all tested specimens. The incubation period of the Ti/TiN-4 coating was 60 min, while in the case of the Ti/TiN-12 and Ti/TiN-40 coatings the incubation periods were shorter than 30 min. At the beginning of cavitation tests (the initial 150 min of exposure), the mass losses of coated specimens were bigger than those of the uncoated specimen. This effect is especially seen in case of the Ti/TiN-40 coating. With the cavitation test duration increase, the mass loss rate of the coated specimens decreased. After the whole cavitation test, the cumulative mass losses of coated specimens were much lower than those of the uncoated specimen. The mass loss rates of the coated specimens were related to the surface roughness and the number of the Ti(N) microdroplets. The microscopic observations suggest that degradation of the coated specimens started by the removal of microdroplets from the surface of the coatings. The mass loss of the Ti/TiN-40 coating, showing the largest amount of microdroplets (the highest Ra parameter), was the highest. Lack of the mass loss of the Ti/TiN-4 coating during the first 60 min of exposure could be due to the lowest number of microdroplets and higher thickness of each layer in this multilayer coating in comparison to the thickness of layers in other multilayer coatings.
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A.K. Krella / Surface & Coatings Technology xxx (2013) xxx–xxx
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Fig. 1. XRD pattern of the Ti/TiN-4 coating.
3.3. Roughness changes Measurements of surface roughness of all tested specimens were performed simultaneously with the mass measurements. The changes of the surface roughness parameter, Ra, with the test duration are shown in Fig. 4. During the first 150 min of the cavitation test the surface roughness of the uncoated stainless steel increased from Ra = 0.02 μm to 1.01 μm and during the rest of cavitation test — by another 1 μm, up to Ra = 1.95 μm. In case of the Ti/TiN-4, Ti/TiN-12 and Ti/TiN-40 coatings, the surfaces roughness increased during the first 150 min of exposure from Ra = 0.3, 0.34 and 0.36 μm up to 0.74, 0.7 and 0.71 μm, respectively. The highest increase of surface roughness parameter in the case of the Ti/TiN-4 coating and the lowest one in the case of the Ti/TiN-40 coating are very symptomatic. During the rest of cavitation tests the surface roughness of the Ti/TiN-4, Ti/TiN-12 and Ti/TiN-40 coatings increased up to Ra = 1.75, 1.29 and 1.26 μm, respectively. These results show that the highest increase in surface roughness of the Ti/TiN-4 coating, close to that of the uncoated steel, is a general trend. The surface roughness of the Ti/TiN-12 coating increased in a similar rate to that of the Ti/TiN-40 coating. Comparing the mass loss curves (Fig. 3) with changes in surface roughness (Fig. 4), some similarity can be noticed. The system with Ti/TiN-4 coating has the lowest mass loss among all tested multilayer coatings and its roughness (Ra parameter) is the closest to that of the uncoated steel. The system with Ti/TiN-40 coating has the highest mass loss among all tested multilayer coatings and the changes in its roughness were the lowest of all. Performed measurements prove that an increase in Ra parameter of the employed systems accompanies a decrease in the mass losses of the systems with multilayer coatings. A similar result was reported in Ref. [5]. The surface profiles of all tested specimens after cavitation tests are presented in Fig. 5. The total height of the profile (the sum of the maximum profile peak height and the maximum profile valley depth over the evaluation length) of the Ti/TiN-40 coating is comparable to that of the Ti/TiN-12 coating and much lower than that of the Ti/TiN-4 coating and the uncoated X6CrNiTi18-10 steel. Moreover, the surface profiles of the Ti/TiN-12 and Ti/TiN-40 coatings show smaller variations that cannot be noticed in the profiles of the Ti/TiN-4 coating and those of the uncoated X6CrNiTi18-10 steel. The mean depth and width of the Ti/TiN-12 and Ti/TiN-40 coating surface
valleys were very close to each other, and equal approximately 3 μm and 150 μm, respectively. The surface roughness profiles of the Ti/TiN-12 and Ti/TiN-40 coatings suggest the similarity in deformation/ degradation process. In the case of the Ti/TiN-4 coating, the mean value of the depth and of the width of surface valleys was 7 μm and 250 μm, respectively. Taking into account that the depth and the width of the valleys of the uncoated steel were 8 μm and 300 μm, respectively, the surface of the Ti/TiN-4 coating clearly resembled that of the uncoated steel. The rough microscopic observations (Fig. 6) confirmed the difference in the deformation between the Ti/TiN multilayer coatings. All the multilayer coatings deformed causing micro-folding. The undulation of the Ti/TiN-4 multilayer coating was most evident. The degree of irregularities decreased with the rising number of layers, which was also obtained using the surface roughness tester. 3.4. Fracture Higher magnification observation has shown that the Ti/TiN multilayer coatings deformed via cracking. First cracks occurred on the top of the micro-folding (Fig. 7). Fig. 7 shows that cracks propagated via simultaneous fracture of many coating layers. Some Ti(N) microdroplets were removed from the surface of the coating, others remained at their original sites. The removed Ti(N) micro-droplets left pits. However, some pits were caused by cavitation microjet penetration. It is difficult to distinguish pits created by the micro-droplet removal and those created by microjets. 4. Discussion 4.1. Properties of coatings The coatings were deposited on the surface of substrate by a condensation process, so the coating bonding to the substrate showed a typical adhesive nature. Because the deposition process proceeded at approximately 350 °C, the thermal properties of the substrate, the Ti coating layer and the TiN layer could influence the thermal mismatch, thermal stresses and the coating adhesion. The difference in thermal expansion between TiN (9.35 μm/m °C) and Ti (8.9 μm/m °C) is low, hence the thermal mismatch and thermal stresses are probably also low. The thermal expansion of the Ti(N) microdroplets is assumed to
Please cite this article as: A.K. Krella, Surf. Coat. Technol. (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.04.016
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A.K. Krella / Surface & Coatings Technology xxx (2013) xxx–xxx
a1
a2
b1
b2
c1
c2
Fig. 2. Cross-sectional micrographs of TiN/Ti multilayer coatings.
be between those of the TiN and the Ti. Thus, the thermal mismatch between the TiN or the Ti layer and the Ti(N) microdroplets should be lower than that between the Ti and the TiN layer. The thermal
expansion coefficient of the substrate (the X6CrNiTi18-10 steel) is 18.70 μm/m °C, which is approximately twice as high as that of Ti. This high difference suggests the risk of high thermal stresses between
Table 3 Properties of Ti/TiN multilayer coatings. Coating
Ti/TiN-4 Ti/TiN-12 Ti/TiN-40
Thickness of coating [μm]
3.7 3.7 3.6
Number of layers
4 12 40
Thickness of single layer [μm]
0.925 0.31 0.09
Hardness of coating [GPa]
Young's modulus of coating [GPa]
Adhesion LC2 [N]
Roughness Ra [μm]
Mean value
Standard deviation
Mean value
Standard deviation
Mean value
Standard deviation
Mean value
Standard deviation
17 16 11
3.14 3.33 3.99
307 348 286
56 108 133
19 19 15
0.87 1.15 1.56
0.3 0.34 0.36
0.02 0.05 0.038
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A.K. Krella / Surface & Coatings Technology xxx (2013) xxx–xxx
7
5
2 X6CrNiTi18-10
5
Ti/TiN-12
1.6
Ti/TiN-40
1.4
4
Ra, um
mass loss, mg
1.8
Ti/TiN-4
6
3
1.2 1 0.8 1H18N9T
0.6
2
Ti/TiN-4
0.4 1
Ti/TiN-12
0.2 0
Ti/TiN-40
0 0
100
200
300
400
500
600
0
100
200
300
400
500
600
time, min
time, min Fig. 3. Cavitation curves.
Fig. 4. Changes of surface roughness with test duration.
the Ti layer and the substrate. Taking into account the risk of thermal mismatch between the multilayer coating and the substrate, low hardness of the X6CrNiTi18-10 steel and its large plastic deformation under substantial pressure of the Rockwell indenter used for the scratch test, the adhesion of the multilayer coatings to the substrate can be considered to be relatively good (Table 3). Because Ti shows the HCP structure with lattice parameters a = 2.95 Å, c = 4.68 Å and TiN shows the FCC structure with lattice parameter a = 4.24 Å, there is a possibility of atomic chemisorptions at the interface between the Ti layer and TiN layer. Moreover, according to Ref. [9], every new layer in a multilayer coating tends to grow in a close structural relationship with the previous layer. Considering low thermal mismatch, the possibility of developing a close structural correlation between the Ti and TiN layers and the possibility of atomic chemisorption between Ti and TiN layers, the adhesion between these two layers should be high. Lugscheider et al. [10] have shown that the microstructure of PVD films, including Arc PVD coatings, changes from a columnar form to a dense and uniform structure with the increase of deposition temperature. TEM investigations of Cr/CrN multilayer coating structure as deposited by the Arc PVD method at 380–420 °C [11] showed that all layers in the multilayer coating had a dense columnar nanocrystalline structure. Also investigations performed by Suresha et al. [12] showed that TiN/(AlTi)N multilayer coatings obtained by the Arc PVD method show columnar microstructure. Moreover, Ref. [12] reports on the columnar growth undisrupted by the TiN/(AlTi)N interface with the columnar grains extending through the entire thickness of the coating. This is in agreement with the Holleck's and Schier's statement [9]. Because titanium nitride and titanium layers were deposited using the Arc PVD method with deposition temperature of 350 °C, so they had columnar nanocrystalline grain structures extending throughout the layer thickness and exhibit anisotropy in elastic and fracture behaviour. Holleck and Schier [9] noticed that an increase in the number of interfaces in the fine grained structures of multilayer PVD coatings often contributes to an increase in crack propagation resistance and toughness due to energy dissipation and stress relaxation at the interfaces. With the increase of number of layers in the employed Ti/TiN multilayer coatings, the number of interfaces increases as well. Moreover, the increase of the number of microdroplets appearing at the coating surface along with the increase of the number of layers in the Ti/TiN multilayer coatings contribute also to the growth of interfaces. Thus, according to Ref. [9], the Ti/TiN-40 multilayer coating should have much better crack propagation resistance than the Ti/TiN-4 one. The performed investigation shows the opposite
relationship, which is probably related to an increase in the number of micro-droplets with the number of layers increasing. Performed investigation of the cavitation erosion resistance shows that more and more microdroplets are removed, leaving empty pinholes, in the beginning of a cavitation test when the number of layers (which is equivalent to the decrease of each layer thickness — Table 3) in the Ti/TiN multilayer coatings increases. It is logical that the low-embedded micro-droplets are removed as the first ones. This investigation shows that the decrease of thickness of each layer in a multilayer coating causes an increase in the number of low-embedded Ti(N) microdroplets. After removing the low-embedded Ti(N) microdroplets the erosion rate falls down. 4.2. Hardness The ability of examined material to deform depends on mechanical properties and its structure. The hardness of Ti/TiN multilayer coatings depends on the ratio of soft titanium to hard titanium nitride contents. Although the amount of soft titanium remained approximately unchanged with the increasing numbers of layers in the employed Ti/TiN multilayer coating, the hardness of the coating decreased (Table 3). Fig. 8 shows the relationship between the thickness of the layers and the hardness of the Ti/TiN multilayer coating. The decrease of layer thickness from 310 nm to 90 nm caused rapid decrease of hardness of the whole Ti/TiN multilayer coating. Santana et al. [13] have used a simple rule of mixture to estimate the hardness of multilayer coating. They have discovered that the calculated hardness of multilayer coating was lower than the measured one. They have concluded that the enhancement of hardness was caused by the interface that had influence on crack propagation. Similar results were obtained by Chen et al. [14]. Applying the rule of mixture to the deposited Ti/TiN multilayer coatings, Santana et al. [13] have determined the following formula (equation for the hardness of the composite coating): Hcomposite ¼
tTi t H þ TiN H ttotal Ti ttotal TiN
ð1Þ
where t is the thickness of each layer and H is its hardness. Taking into account that the thickness of all Ti layers and all TiN layers remained nearly the same, the calculated multilayer hardness should be unchanged for all multilayer coatings despite the increasing number of layers. The average hardness of the coating deposited in our experiments, determined from Eq. (1) is 10.9 GPa. Assuming that the interface enhances the hardness of multilayer coating, as it
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A.K. Krella / Surface & Coatings Technology xxx (2013) xxx–xxx
15.0 10.0
a)
X6CrNiTI18-10
[um]
5.0 0.0 -5.0 -10.0 -15.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
[mm] 15.0 10.0
b)
Ti/TiN-4
[um]
5.0 0.0 -5.0 -10.0 -15.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
[mm] 15.0 10.0
c)
Ti/TiN-12
[um]
5.0 0.0 -5.0 -10.0 -15.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
[mm] 15.0
d)
Ti/TiN-40
10.0
[um]
5.0 0.0 -5.0 -10.0 -15.0 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
[mm] Fig. 5. Roughness profiles of tested specimens after cavitation test; a) the profile of the X6CrNiTi18-10 steel, b) the profile of the Ti/TiN-4 multilayer coating, c) the profile of the Ti/TiN-12 multilayer coating, and d) the profile of the Ti/TiN-40 multilayer coating.
was proved in Ref. [13,14], the hardness of the coatings should increase with increasing number of layers. However, the measurements of hardness in our experiments have shown an opposite rule. The hardness of the Ti/TiN-40 multilayer coating was the smallest and close to the calculated value, and the hardness of the Ti/TiN-4 multilayer coating was the highest. It can be supposed that micro-droplets might be responsible for the decreasing hardness of the multilayer coatings. Performed investigations showed that with increasing number of layers, the number of micro-droplets per unit surface also increases. Nevertheless, an increase of the number of micro-droplets was not as clear as the drop of hardness. With increasing roughness of the coating from
0.30 μm for the Ti/TiN-4 coating to 0.34 μm for the Ti/TiN-12 coating and to 0.36 μm for the Ti/TiN-40 coating, the hardness dropped from 17 GPa for the Ti/TiN-4 coating to 16 GPa for the Ti/TiN-12 coating and to 11 GPa for the Ti/TiN-40 coating. In the case of the Ti/TiN-12 coating, the number of interlayers increased from 4 to 12 and the roughness increased from 0.30 to 0.34 μm, while, as a result, the hardness decreased by 1 GPa. In case of the Ti/TiN-40 coating, the number of interlayers was 40, but compared to Ti/TiN-12 coating the roughnesses differ only slightly from 0.34 μm (Ti/TiN-12) to 0.36 μm. Although following the results of Santana et al. [13] and Chen et al. [14] it could be expected that the hardness should not change significantly, in fact,
Please cite this article as: A.K. Krella, Surf. Coat. Technol. (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.04.016
A.K. Krella / Surface & Coatings Technology xxx (2013) xxx–xxx
Ti / TiN-4
7
a)
b) Ti / TiN-12
c) Ti / TiN-40
Fig. 7. Surface profiles of degraded specimen after cavitation test; a) Ti/TiN-4 coating, b) Ti/TiN-12 coating, and c) Ti/TiN-40 coating.
Fig. 6. Surface profiles of tested specimens after cavitation test.
it decreased rapidly from 17 GPa to 11 GPa. This result shows that the influence of the interfaces on the multilayer coating hardness is not as simple as it was proved by Santana et al. [13] and Chen et al. [14]. In present investigations TiN, whose hardness is higher than that of Ti, was always the outer layer. The indentation depth during the measurements of coating hardness should not exceed 1/10 of the coating thickness. In present measurements the maximum indentation depth was 200 nm. The thickness of the TiN top layer in the Ti/TiN-4 coating was approximately 920 nm. Taking into account the indentation depth and the thickness of top layer, the conclusion can be that the hardness of the top layer dominated the hardness of the whole multilayer coating. In case of the Ti/TiN-12 coating, the thickness of the top TiN layer was approximately 300 nm. So, the
indentation depth was nearly the same as the thickness of the top layer. Thus, the soft Ti layer just under the TiN layer contributed significantly to the total hardness. In case of the Ti/TiN-40 coating, the thickness of the top TiN layer was approximately 90 nm. The indenter during hardness measurements penetrated through the whole thickness of the TiN and Ti layer. Thus, the influence of the soft Ti layer on the total hardness increased. Probably, therefore, the hardness of this coating was close to the calculated hardness. Performed investigations suggest that the hardness and thickness of outer layer have significant influence on the hardness of the whole multilayer coating. 4.3. Fracture Performed cavitation erosion tests have shown good protective properties of all tested Ti/TiN multilayer coatings (Fig. 3). Nevertheless, the protective properties lessened with the decrease of the layer thickness (decrease of the coating hardness) (Fig. 9). Correlation between
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A.K. Krella / Surface & Coatings Technology xxx (2013) xxx–xxx
Fig. 8. Correlation between coating hardness and thickness of layers in TiN/Ti coatings.
the mass loss and the coating hardness was investigated also in Ref. [5], where the influence of mechanical properties of TiN monolayer coatings, as deposited at different technological parameters, on cavitation erosion resistance was investigated. Ref. [5] shows that for the TiN monolayer coating there exists a critical value of hardness. An increase of hardness in the subcritical domain results in a decrease of the mass loss whereas mass loss increase is accompanied by the rise of the coating hardness for the supercritical hardness values. This was explained by the absorption of energy being used for the deformation of the TiN monolayer coating, crack generation and their propagation [5]. In this study, the increase of the mass loss with the decrease of hardness of the multilayer coating can be also related to the absorption of energy on deformation and crack initiation, especially by the Ti layers, the Ti(N) microdroplets and the interfaces. To analyse the process of material degradation by cavitation, it is important to know the factors that cause the damage. Cavitation erosion is caused by repeated action of cavitation bubbles collapsing in the vicinity of the solid surface. The high speed camera observations of the cavitation bubbles collapse [15–17] which have revealed that the bubbles lose their spherical shape, and emit shock waves and microjets. Measurements of material loading caused by cavitation bubbles [16–20] show a wide range of amplitude from tens of kPa up to hundreds of MPa. Generally, the microjets are directed towards the solid surface [15–17], such direction of microjet has been assumed also in the present analysis.
Fig. 9. Correlation between mass loss and hardness of TiN/Ti coatings.
Due to the wide range of microjet speed and amplitude of the impacting shock waves [15–22], the solid material is passing locally from elastic deformation to adiabatic shearing. Such loading may activate many deformation mechanisms. Due to assumed columnar structure of the Ti/TiN multilayer coating, shear cracking and sliding along columnar grain boundaries are the easiest deformation mechanisms for such materials. Due to the TiN structure resembles that of the NaCl, dislocations show high activation energy. On the other hand, activation energy decreases in nanocrystalline ceramics [23]. Investigations presented in Ref. [23,24] show that during the deposition process dislocations were generated in the TiN grains and the external stress was able to induce dislocation motion at room temperature. For this reason it is assumed that dislocations occur in TiN layer, deposited by Arc PVD, and deformation mechanisms such as dislocation motion, sliding/shearing along grain boundaries, and shear band formation can be activated by cavitation pulses. In the case of Ti grains, similar deformation mechanisms are activated, but because the size of Ti grains is approximately three times bigger than that of the TiN grains, and due to low dislocation activation energy, the contribution of dislocation motion to the whole deformation is expected to be larger than that in the TiN grains. The microjet impacts the surface of the coating with high impact speed. The coating is compressed down in a single direction. The generated stresses propagate within the coating as a wave, but each activated mechanism needs some energy, so finally the stresses and waves disappear. In general, the dislocations are forced to motion if stress exceeds some critical level, which increases with rising strain rate and decreases with rising temperature. In the TiN grains the stress needed to move a dislocation, σyTiN, is high due to the ionic bonding, while stress needed to cause sliding/shearing along grain boundary, σgbTiN, is much lower than σyTiN due to columnar structure of the grains. In the same way, the stress needed to move dislocation in Ti grains, σyTi, is higher than the stress needed to cause sliding/ shearing along grain boundary, σgbTi. Moreover, the stresses σyTiN and σgbTiN are higher than σyTi and σgbTi, respectively, due to the TiN and Ti bonding nature. When external stress, σ, is applied to the TiN grains, it runs mainly through the grain boundary, because σgbTiN ≪ σyTiN. If the impact energy is high enough, then not only sliding but also fracture along grain boundary occurs. Crack will propagate throughout the TiN layer. On the other hand, the investigation of Kumar et al. [23,24] has indicated that the dislocation motion in the TiN grains under external stress is possible. Because of high σyTiN, some stresses that run in the plane perpendicular to the impact plane release activating dislocations — some of them are blocked by obstacles, e.g. anchored dislocation, increasing the stress level in the TiN grains. To summarize, some of impact stresses cause dislocation motion inside the grains, but most of them cause sliding and fracture along grain boundaries. Because of the columnar structure of the grains, sliding along grain boundary is the dominating deformation mechanism in the Ti layer. Due to the size of the grains and the rate of impact, the shear bands may be created. The dislocation motion is also expected to occur. Assuming that stress concentration at the crack tip is bigger than σyTi, a plastically deformed zone is generated in grains as a result of the dislocation motion. On the other hand, due to high stress wave velocity, the shear bands may be initiated inside the Ti grains. Consequently, some impact energy is absorbed and the level of the shock wave that induced stresses decreases. If the remaining impact energy is high enough, the crack continues to propagate through the Ti layer to the Ti–TiN interface, where it may be halted temporarily. Besides this, the deformed Ti grains compress the underneath TiN grains. Under this pressure, if the exerted stress σ is higher than σgbTiN, sliding along grain boundary in the TiN grains can proceed. If the impact energy is still high enough, fracture/crack along grain boundary is initiated. Energy absorbed in each layer by means of all deformation mechanisms is related to the layer mechanical properties and thickness.
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The decrease of layer thickness from 0.925 μm to 90 nm results in a drop of energy absorption. As a result, more energy from impact of each microjet will reach the deeper layers causing their fracture. Therefore, fractures observed at the Ti/TiN-40 and Ti/TiN-12 coatings (Figs. 7 and 8) were several layers deep. The character of bonding of the Ti(N) microdroplet to the Ti layer is different from that to the TiN layer. As a result, the bonding of a Ti(N) microdroplet to the coating becomes weakened along with the decrease of the thickness of each layer. Under attack of cavitation microjets, the Ti(N) microdroplets are easily removed when the thickness of the layers decreases because of weakening their bonding to the coating and increasing microjet action depth. Thus, with the decrease of layer thickness more microdroplets are removed, and their removal occurs sooner. In the present experiments it was observed that the mass loss of the Ti/TiN-40 coating under cavitation test conditions was initiated without any incubation period and the total mass loss after the whole test was higher than that of the Ti/TiN-4 coating (Fig. 3). Another effect of the layer thickness is a possibility of deformation of the Ti/TiN-40 coating generated by impacts of low-energy cavitation microjets. Undulation of surface profiles of the Ti/TiN-40 and the Ti/TiN-12 coatings as visible in Fig. 5, was probably caused by the impacts of cavitation microjets of lower energy, which were not able to deform the thicker Ti/TiN-4 coating layers. The Ti/TiN-4 coating was deformed by impacts of microjets of higher energy. Therefore, the roughness of the surface on the Ti/TiN-4 coating was higher than that on the Ti/TiN-12 and Ti/TiN-40 coating surface. 5. Conclusions This study presents the results of cavitation tests of Ti/TiN multilayer coatings deposited on the X6CrNiTi18-10 austenitic steel carried out by means of the ARC PVD method. The obtained results and their analysis led to the following conclusions: • Deposition of Ti/TiN multilayer coatings improves cavitation erosion resistance of the protected surface. • An increase in the number of Ti(N) droplets within the coating increasing the coating roughness causes an increase of the mass loss rate at the beginning of a cavitation. • The mass loss of the Ti/TiN multilayer coatings decreases with an increase of the coating hardness and an increase of thickness of the layers. • Hardness and thickness of top layer have influence on the hardness of the multilayer coating. • All multilayer coatings underwent micro-folding at the early stage of cavitation erosion. The deformation developed via cracking, which
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propagated by simultaneous fracture of many coating layers. The first micro-cracks appeared at the top of undulation. • Energy absorbed in each layer due to all deformation mechanisms depends on the grain structure, bonding nature, mechanical properties and thickness of the layer.
Acknowledgements The author thanks warmly Dr. Andrzej Czyzniewski, Koszalin University of Technology, Institute of Mechatronics, Nanotechnology and Vacuum Technique, for deposing the Ti/TiN multilayer coatings, and performing cross-sectional micrographs and adhesion measurements. The author thanks warmly Prof. Maria Gazda, Gdansk University of Technology, Department of Solid State Physics, for performing the X-Ray diffraction pattern of the Ti/TiN-4 coating. References [1] Y.H. Cheng, T. Browne, B. Heckerman, C. Bowman, V. Gorokhovsky, E.I. Meletis, Surf. Coat. Technol. 205 (2010) 146. [2] M. Bromark, M. Larsson, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol. 90 (1997) 217. [3] U. Wiklund, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol. 97 (1997) 773. [4] X.H. Zhang, D.X. Liu, H.B. Tan, X.F. Wang, Surf. Coat. Technol. 203 (2009) 2315. [5] A. Krella, Surf. Coat. Technol. 204 (2009) 263. [6] A. Vojvodic, C. Ruberto, B.I. Lundqvist, Surf. Sci. 600 (2006) 3619. [7] R.M. Wood, Proc. Phys. Soc. 80 (1962) 783. [8] S. Münsterer, K. Kohlhof, Surf. Coat. Technol. 74–75 (1995) 642. [9] H. Holleck, V. Schier, Surf. Coat. Technol. 76–77 (1995) 328. [10] E. Lugscheider, C. Barimani, C. Wolff, S. Guerreiro, G. Doepper, Surf. Coat. Technol. 86–87 (1996) 177. [11] P. Wieciński, J. Smolik, H. Garbacz, K.J. Kurzydłowski, Thin Solid Films 519 (2011) 4069. [12] S.J. Suresha, R. Bhide, V. Jayaram, S.K. Biswas, Mater. Sci. Eng., A 429 (2006) 252. [13] A.E. Santana, A. Karimi, V.H. Derflinger, A. Schutze, Surf. Coat. Technol. 177–178 (2004) 334. [14] L. Chen, S.Q. Wang, S.Z. Zhou, J. Li, Y.Z. Zhang, Int. J. Refract. Met. Hard Mater. 26 (2008) 456. [15] A. Philipp, W. Lauterborn, J. Fluid Mech. 361 (1998) 75. [16] I.R. Jones, D.H. Edwards, J. Fluid Mech. 7 (1960) 596. [17] Ch.E. Brennen, Oxford Engineering Science Series, 44, Oxford University Press, 1995, p. 73, (80). [18] A. Krella, A. Czyżniewski, Wear 260 (2006) 1324. [19] T. Okada, Y. Iwai, S. Hattori, N. Tanimura, Wear 184 (1995) 231. [20] S. Hattori, T. Hirose, K. Sugiyama, Wear 269 (2010) 507. [21] M.S. Plesset, A. Prosperetti, J. Fluid Mech. 9 (1977) 145. [22] N.K. Bourne, Shock Waves 11 (2002) 447. [23] S. Kumar, D.E. Wolfe, M.A. Haque, Int. J. Plast. 27 (2011) 739. [24] S. Kumar, D. Zhuo, D.E. Wolfe, J.A. Eades, M.A. Haque, Scr. Mater. 63 (2010) 196.
Please cite this article as: A.K. Krella, Surf. Coat. Technol. (2013), http://dx.doi.org/10.1016/j.surfcoat.2013.04.016