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Growth defects in PVD hard coatings
Vacuum 84 (2010) 209–214
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Growth defects in PVD hard coatings P. Panjan*, M. Cekada, M. Panjan, D. Kek-Merl Jo zef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
a b s t r a c t Keywords: Growth defects PVD hard coatings 3D stylus profilometry Focused ion beam
In PVD coatings, various growth defects typically appear during the deposition. Such defects are drawbacks in coating application. In order to improve the tribological properties of PVD hard coatings it is important to minimize the defect density. Various PVD hard coatings were prepared by evaporation using a thermionic arc and by sputtering using unbalanced magnetron sources. Coating topography was analyzed using a 3D stylus profilometer and other analytical techniques (SEM, FIB). We studied the influence of different types of substrate materials, the substrate position in the vacuum chamber, pretreatment and deposition parameters on defect density. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Coated surfaces are never perfect. Besides grooves and ridges the surface also displays pronounced conical features, pin-holes, pores and other open voids which form during the deposition process [1–8]. The defects are non-uniformly distributed over the coating surface and depth, while their form, size and density depend on deposition conditions. They are induced either by substrate irregularities (pits, asperities), foreign particles (dust, debris, polishing residues) or by the coating deposition process (incorporation of small particles and microdroplets). Vetter et al. [1] classified the growth defects in sputter-deposited a-C films as through-voids, close voids, flake formation, overcoated particles and those due to the coarse columnar morphology. Flakes are generated when some foreign particles are at the substrate surface before the coating starts to grow, or they are built into the growing coating. The voids can be interpreted as ‘‘inverse’’ flakes, generated by a lifting off of the flakes. They found that the main factors which influence defect density are the cleanness of the chamber, the ion cleaning step, the sputtering power and shielding of the substrates. They reported that the defect density was reduced by approximately a factor of 10 when they used an additional shielding of the substrates (shield surrounded all substrate surfaces except the substrate front surfaces). Further reduction of the defect density was observed if they worked at a lower substrate temperature during etching and deposition. Aharonov et al. [2] investigated the origin of the growth defects on the surface of arc evaporated hard coatings. They studied the
effect of bias voltage characteristics (amplitude for d.c. voltage, frequency and duty cycle (for pulsed voltage)) on formation of defects. They found that an increase in the amplitude or duty cycle resulted in a decrease of the defect density, roughness and the surface area covered by the defects. They explained these phenomena by this increase of energy level of impinging ions, thus increasing the adatom mobility and surface diffusion. An additional mechanism could be the higher electric field density on the conical features resulting in a higher resputtering rate. Growth defects are a drawback in hard coating applications. The incorporation of macroparticles into a coating changes significantly the surface morphology and roughness. Consequently the contact area increases and thus friction increases too. Additionally, the pinholes created by debonding of micro- and macroparticles have a deleterious effect on the corrosion resistance and gas permeation of the coatings. Growth defects also cause higher sticking of workpiece material. In order to improve the tribological properties of PVD hard coatings it is important to minimize the surface concentration of defects. However, the important question to answer is how to deposit a denser and less defective coating. In our previous works [6–8] we used several analytical techniques for defect analysis. In this work, we present the results of coating topography investigation by means of 3D stylus profilometry. The field emission scanning electron microscope (SEM) was also used to study of the defect morphology. An SEM in combination with a focused ion beam technique (FIB) and atomic force microscopy (AFM) analyses were performed on selected samples. 2. Experimental
* Corresponding author. E-mail address: [email protected] (P. Panjan). 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.05.018
The BAI 730M (Balzers) deposition system with a thermionic arc was used for deposition of TiN and CrN, while the magnetron
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sputtering system CC800/7 (CemeCon) was used for deposition of TiAlN, TiN, CrN and CrN/TiAlN multilayer hard coatings. Four types of substrates were used: a powder metallurgical high speed steel ASP30, a hot work tool steel H11, a cold work tool steel D2 and cemented carbide. The substrates were polished, ultrasonically cleaned and dried in hot air. Prior to deposition they were cleaned by ion etching. In the BAI 730 system, the thermionic arc in a triode arrangement is used for heating, etching and evaporation process steps. Prior to deposition, the substrates were heated to 450 C. They were then Ar ion sputter-cleaned for 15 min with an applied substrate bias of 200 V. During deposition the bias voltage was 125 V and the deposition rate of TiN and CrN hard coatings was about 50 nm/min. The CC800 system is equipped with four 8 kW unbalanced magnetron sources. During substrate cleaning an RF bias was applied (maximum RF power was 2 kW), while the etching time was 85 min. The substrate temperature was about 450 C. During deposition the bias voltage was 100 V. During deposition of TiAlN, TiN and CrN coatings the power on targets was 8 kW, 8 kW and 3 kW, respectively. During deposition the substrates undergo one-, two- or threeaxis planetary rotation. The surface morphology of the coatings was examined using a 3D stylus profilometer (Taylor Hobson Talysurf). Its vertical resolution was a few nm, while the lateral resolution was limited to 1 mm. The measured area was 1 mm 1 mm. A field emission scanning electron microscope (Zeiss Supra 35 VP) was used for examination of the morphology and microstructure of coating in planar surface view and cross-sectional fracture view. A focused ion beam (FIB) workstation was used to prepare a cross-section through the defects. We used FIB integrated in the FEI QUANTA 200 3D microscope. An ion beam was used to remove precise sections of material (close to the defect) from the specimen surface by sputtering. 3. Results and discussion
Fig. 1. Optical micrograph of the ball crater on the ASP30 substrate coated with a 8 mm thick TiN. Black dots on ground section are the cross sections of defects.
compressive stresses in deposit increases linearly with deposition time. At the beginning of the deposition process such stresses in the deposit were small while, after previous deposition, the critical deposit material was removed due to thermal stresses during cooling of the chamber. After a certain deposition time the compressive stresses in the deposit exceeded once again the critical values and the delamination process became more intense. 3.2. SEM and FIB study of defects Field emission SEM was used for examination of the morphology and microstructure of coatings in planar surface view and cross-sectional fracture view. Hard coating morphology can be treated on several levels:
3.1. Defect study by optical microscopy In the optical images the defects are seen as black dots. However, in the optical image the peaks (protrusions) and holes cannot be distinguished. In addition, lateral and depth resolution is not good enough for detailed analysis of defects. By comparison of optical images of the same area of substrate surface before and after deposition we can observe that some of the defects after deposition remain at the same position, some of them disappear, while some additional new defects appear. Based on this we can conclude that the origin of one part of growth defects arises from pretreatment of the substrate surface before deposition, while the second group of defects appears during deposition. Using appropriate software for image processing, it is possible to perform qualitative and quantitative analysis of defects. The optical microscopy was used also in combination with a ballcratering technique. The ball-cratering preparation technique was employed to prepare a low-angle cross-section of the coating. The concentration of defects on the ground section was observed by an optical microscope. Fig. 1 shows an optical micrograph of the ball crater on the ASP30 substrate coated with an 8 mm thick TiN (coating thickness t was determined by equation t ¼ (Y2–X2)/8R, where X and Y are inner and outer diameters of the circle and R the ball radius). It is evident that the concentration of defects is much higher close to the top surface while there are much fewer defects at the substratecoating interface. This means that most defects appeared after a certain deposition time. Our explanation is that the delamination of small particles from chamber components (shields, fixturing) is much more frequent with the deposition time, because the
- nanoscale: the morphology is determined by structural growth defects - submicroscale: the morphology is determined by nucleation mechanisms. Depending on growth conditions coarse columnar, fine columnar or amorphous growth modes are possible. The coating surface morphology is determined by the top morphology of columns. - microscale: morphological features are determined by growth defects (voids, flake, overcoated particles, pin-holes) - macroscale: the morphology is determined by the topography of bare substrate (scratches, pits, grooves, ridges). All morphological features of bare substrates are transferred and magnified through the coating. Thus on the SEM plan view of coatings we can clearly see the hillocks and pits which are at the position of carbide inclusions in the substrate. It is well known that hillocks and pits are generated by polishing and ion etching at the position of carbide inclusions. On Fig. 2a, which shows the surface morphology of TiAlN coating on powder metallurgical tool steel ASP30 substrate, a circular flat-topped area (approximately 1 mm in diameter) corresponding to overcoated carbides are visible. Similar features were observed on coated D2 tool steel (Fig. 2b). Their shape reflected the irregular form of chromium carbides in the steel matrix (typical dimension between 3 and 15 mm). We did not observe such morphological features on hot working tool steel H11 and cemented-carbide substrates. The other morphological features on the microscale are determined by growth defects. They can be classified as follows:
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Fig. 2. Plan view (a,b,c,d,h), cross-sectional SEM (e,g,i,j,k) and FIB images (f,l) of the following types of defects: (a) circular flat-topped morphological features at carbide inclusions in ASP30 tool steel (b) irregular flat-topped morphological features at carbide inclusions in D2 tool steel, (c) nodular or flake defect, (d) foreign particles preventing etching of the surface covered by them (e) cross-section of flake defect (f) FIB image of flake cross-section, (g,h) through-voids or dish-like craters (i,j) cone-like defects (k) SEM image of pin-hole fracture cross-section (l) FIB image of pin-hole cross-section.
(a) Defects in the form of flakes Nodular or flake defects are large (typical dimension is 5–40 mm) and irregularly shaped (Fig. 2c). They are loosely-bound defects, which form at asperities, debris and other foreign particles. Such particles come either from the vacuum chamber (heaters, shields and other components) or they stick to the substrate surface before the deposition process (metal dust, debris, polishing residues). Surface imperfections are preferred nucleation sites at which the coating is growing more rapidly than in the bulk deposit. Foreign particles also prevent etching of the substrate surface covered by them (Fig. 2d). The shadowing effect of such particles results in weak bonding between the overcoated particle and the surrounding coating. Therefore they can be easily detached due to the internal or thermal stresses. The coated particle could be also deformed. In this case a gap between the coating and the defect appears which can serve as crack initiators (Fig. 2e). Fig. 2f shows a FIB image in cross-section of the flake defect. In this case the foreign flake particle was built into the coating after a certain deposition time. EDX analysis revealed that its composition is iron-based.
(b) Open void defects The coated foreign particles are loosely bound to the coating and, during deposition, a part of them can spontaneously flake
off due to high compressive stress in the coating above critical thickness. Part of them flakes off during the cooling of the vacuum chamber due to thermal stress. This leads to the formation of through-voids (Fig. 2g). Actually voids can be interpreted as ‘‘inverse’’ flakes. If the flake is detached during deposition, the crater surface is covered by a thinner layer of hard coating with rough and porous features (Fig. 2h). (c) Cone-like defects These defects are smaller and have round, cone-like shape with a diameter of approximately 1 mm. They originate from fine particles (100–400 nm), which flake from the vacuum chamber components. On their way through plasma they can get charged. Such particles are transported to the substrate and they will be held to it by an electrostatic force, which is much stronger than gravity. During the deposition process they can be built into the coating forming cone-like defects. The flaking of such small particles can be caused by thermal and internal stresses in deposits on chamber components. As explained before, the number of such particles increases with deposition time. The defects which originate from such inclusions look like a cone in cross-section (Fig. 2i). Small foreign particles of submicrometer size which are built into a coating are difficult to identify from a cross-sectional fracture SEM image because the composition of dust defects is usually the same as composition of the coating,
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but they can be clearly visible in multilayer hard coatings. From the layer contour form we can find the positions at which such particles are built into the coating (Fig. 2j).
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(d) Pin-hole defects Defects, in the form of pin-holes extend through the whole coating (Fig. 2k, l). They are generated at the substrate pits (cavities) mostly formed during polishing [7]. Those cavities which are too narrow cannot be covered completely by a relatively thin hard coating due to a shadowing effect and, the coating is preferentially deposited to the flat front side of the substrate, while the deposition rate on the sidewall of the cavity is much smaller. The result is the formation of pin-holes. In smaller holes the opening is closed after a certain deposition time and a void is formed below it. Larger holes are never closed. Due to the shadowing effect, coating on the sidewalls of the hole has a columnar, porous structure.
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Growth defects of various sizes are typically non-uniformly distributed over the coating surface and their surface density is rather low (typically several tens up to a thousand defects per mm2). We found that the portion of the coating surface covered by defects is 0.5–4%. In order to obtain the correct defect density we have to analyze a relatively large surface area. For this purpose we used a 3D stylus profilometer, which has a very good lateral (0.25 mm in the xdirection, 1 mm in the y-direction) and vertical resolution (approximately 5 nm). Thus we obtained a 3D image (Fig. 3a,b) of the coating surface on a large scanning area with all the micrometer-sized details. The initial result of the 3D stylus profilometry is a map z(x,y) over the scanning range. Some corrections have to be applied to level the profile, exclude the general geometrical shape or possible measurement-induced misfits. Software for surface analysis usually gives several parameters regarding the peak (or hole) statistics: number of peaks, average peak height, average peak volume, average peak surface, total volume of peaks, maximum peak height, surface roughness, etc. A threshold value of what is to be counted as a peak or a hole is necessary for some of these parameters. We performed 3D profile measurements and detailed analysis on a series of different samples. Each sample was measured twice and each analysis was performed separately. The thresholds for peaks and craters were 0.5 mm and 0.25 mm, respectively. We selected a lower threshold for holes, because due to the conical shape of the stylus tip the depth of narrow holes can be underestimated. The stylus tip has a radius of curvature of 1.5–2.5 mm, and the cone angle is 90 . From the 3D image we estimated the surface density and height distribution of the defects. The influence of PVD deposition technique, deposition time, deposition rate, substrate type and its
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(e) Defects in the form of microdroplets Such types of defect are a consequence of arcing. All impurities on targets and substrates increase the arc tendency during etching or deposition steps. Arcs can cause the emission of microdroplets that are incorporated into a coating during its growth. Such defects are frequent especially in the case of cathodic arc deposition. We rarely find such kind of defects in hard coatings prepared by ion plating with thermoionic arc or by sputter deposition. However, we do not preclude the indirect influence of arcing on the formation of growth defects. Due to the high energy which arises during arcing at certain points of the substrate table, anode and other components of the vacuum chamber, we can expect that locally high thermal stresses appear. Such stresses could be one of the reasons for delamination of deposited coatings.
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location and orientation in the chamber on defect density was also analyzed. The result of 3D profile measurements are: - The reproducibility for the same sample is rather good. We observed that the number of peaks decreases by approximately 10% after the second measurements on the same surface area, while the number of craters increases for the same value. The reason could be the removal of some loosely bound coated particles by stylus tip and formation of craters. - The 3D profile image of the substrate surface after cleaning and after etching did not show any peaks, while the tip removes the particles which are on the substrate surface. Therefore it could not detect loosely-bound particles on bare substrates after cleaning and ion etching, although they were observed with an optical microscope. 3D profile images of such substrates show only a low density of craters (pits). It is well known that such defects appear already during mechanical pretreatment of tool steel substrates where some carbide inclusions could be torn out of material during polishing, leaving pits on the surface. - We found a correlation between the peak density and the crater density (Fig. 4a). The correlation is stronger for sputterdeposited coatings than for the evaporated ones. This can be explained by partial transformation of peaks into craters (part of coated foreign particles flake off due to the stresses). This process is more intensive for sputtered than for evaporated coatings due to the higher compressive stresses. - The number of peaks exceeds the number of craters by an order of magnitude (Fig. 4b). This means that only a smaller number of peaks (flakes) transformed into a craters. - Peaks may be several times higher than the film thickness, while the craters can be, at most, as deep as the film. We
P. Panjan et al. / Vacuum 84 (2010) 209–214
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Fig. 4. (a) The correlation between the peak density and the crater density for TiAlN coatings deposited on ASP30 tool steel and cemented carbide substrate (HM) (b) The peak and crater densities for a set of TiAlN coatings (64 samples) deposited on cemented carbide substrates (all measurements are presented) (c) distribution of peak density in dependence of its height for two TiN coatings of different thickness (3,9 mm and 5,8 mm) prepared by sputter deposition (d) relation between the density of peak and surface roughness.
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observed also that the higher is the defect density, the higher are the peaks and the deeper are the craters (Fig. 4c). There is also a good correlation between the defect density and surface roughness Sa. As was expected Sa increases with increasing defect density (Fig. 4d). A weak correlation, between the roughness and frequency of arcs on the substrate table was observed. A possible mechanism which is responsible for this correlation is explained in paragraph 3.2e. If we compare the samples of the same batch (the same deposition time) than there is no correlation between the coating thickness (it depends on substrate position and its rotation mode) and the defect density. This means that the same number of defects were built in the coatings irrespective of its thickness. The defect density depends on deposition time. The longer the times, the more defects are built into the coating. The density of defects depends on the position of the substrate in the deposition chamber. If the substrate is in a horizontal position (substrate surface is perpendicular to the target surface), then the defect density is two times larger on the top side of the substrate than on the bottom one. There is a clear trend of decreasing defect density for a sample in a vertical position (substrate surface is parallel to the on target surface). It is higher on the substrate at the bottom of the vacuum chamber, lower at top). Another parameter is the orientation of the sample. The vertically-mounted samples have a lower defect density than the horizontally mounted ones. The same coatings deposited on different substrate materials are not distinguished by difference in defect density. However, we found that the increase of surface roughness is different for different substrates. This could be explained by different ion etching effects. Different tool steels are composed of carbides
of various sizes and composition embedded in the matrix. The sputtering rates of these carbides and the matrix vary. This results in formation of a large number of pits and hillocks, while the surface roughness increases. - Different hard coatings prepared by the same deposition technique are distinguished by the number of defects. We found the lowest number of defects in sputter-deposited CrN coatings, more in TiAlN, while most defects are found, in the TiN coating. - Coatings prepared by different deposition techniques are distinguished by the defect density. The coatings deposited by thermoionic ion plating are, in general, smoother in comparison to the sputter-deposited coatings.
4. Conclusions We used 3D profilometry to estimate the surface density and height distribution of the defects. We found that defect density depends on deposition time, deposition technique, deposition parameters, substrate position in the vacuum chamber, its orientation and rotation mode. We did not find any correlation of defect density with coating thickness and substrate material, while the correlation of defect density with arcing was not very pronounced. We have also shown that SEM in combination with FIB provide a very useful technique for structural and compositional analysis of selected growth defects.
Acknowledgements This work was supported by the Slovenian Research Agency (project L2-9189).
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References [1] Vetter J, Stu¨ber M, Ulrich S. Surf Coat Technol 2003;168(2–3):169. [2] Aharonov RR, Chhowalla M, Dhar S, Fontana RP. Surf Coat Technol 1996;82(3):334. [3] Mu¨nz WD, Lewis DB, Creasey S, Hurkmans T, Trinh T, Ijzendorn W. Vacuum 1995;46:323. [4] Wang HV, Stack MM, Lyon SB, Hovsepian P, Mu¨nz WD. Surf Coat Technol 2000;135:82.
[5] Lewis DB, Creasey SJ, Wu¨stefeld C, Ehiasarian AP, Hovespian PE. Thin Solid Films 2006;503:143. [6] Panjan P, Kek-Merl D, Zupanicˇ F, Cekada M, Panjan M. Surf Coat Technol 2008;202(11):2302. [7] Cekada M, Panjan P, Kek-Merl D, Panjan M, Kapun G. Vacuum 2007;82(2):252. [8] Kek-Merl D, Panjan P, Panjan M, Cekada M. Plasma Process Polym 2007;4:5613.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Growth defects in PVD hard coatings P. Panjan*, M. Cekada, M. Panjan, D. Kek-Merl Jo zef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
a b s t r a c t Keywords: Growth defects PVD hard coatings 3D stylus profilometry Focused ion beam
In PVD coatings, various growth defects typically appear during the deposition. Such defects are drawbacks in coating application. In order to improve the tribological properties of PVD hard coatings it is important to minimize the defect density. Various PVD hard coatings were prepared by evaporation using a thermionic arc and by sputtering using unbalanced magnetron sources. Coating topography was analyzed using a 3D stylus profilometer and other analytical techniques (SEM, FIB). We studied the influence of different types of substrate materials, the substrate position in the vacuum chamber, pretreatment and deposition parameters on defect density. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Coated surfaces are never perfect. Besides grooves and ridges the surface also displays pronounced conical features, pin-holes, pores and other open voids which form during the deposition process [1–8]. The defects are non-uniformly distributed over the coating surface and depth, while their form, size and density depend on deposition conditions. They are induced either by substrate irregularities (pits, asperities), foreign particles (dust, debris, polishing residues) or by the coating deposition process (incorporation of small particles and microdroplets). Vetter et al. [1] classified the growth defects in sputter-deposited a-C films as through-voids, close voids, flake formation, overcoated particles and those due to the coarse columnar morphology. Flakes are generated when some foreign particles are at the substrate surface before the coating starts to grow, or they are built into the growing coating. The voids can be interpreted as ‘‘inverse’’ flakes, generated by a lifting off of the flakes. They found that the main factors which influence defect density are the cleanness of the chamber, the ion cleaning step, the sputtering power and shielding of the substrates. They reported that the defect density was reduced by approximately a factor of 10 when they used an additional shielding of the substrates (shield surrounded all substrate surfaces except the substrate front surfaces). Further reduction of the defect density was observed if they worked at a lower substrate temperature during etching and deposition. Aharonov et al. [2] investigated the origin of the growth defects on the surface of arc evaporated hard coatings. They studied the
effect of bias voltage characteristics (amplitude for d.c. voltage, frequency and duty cycle (for pulsed voltage)) on formation of defects. They found that an increase in the amplitude or duty cycle resulted in a decrease of the defect density, roughness and the surface area covered by the defects. They explained these phenomena by this increase of energy level of impinging ions, thus increasing the adatom mobility and surface diffusion. An additional mechanism could be the higher electric field density on the conical features resulting in a higher resputtering rate. Growth defects are a drawback in hard coating applications. The incorporation of macroparticles into a coating changes significantly the surface morphology and roughness. Consequently the contact area increases and thus friction increases too. Additionally, the pinholes created by debonding of micro- and macroparticles have a deleterious effect on the corrosion resistance and gas permeation of the coatings. Growth defects also cause higher sticking of workpiece material. In order to improve the tribological properties of PVD hard coatings it is important to minimize the surface concentration of defects. However, the important question to answer is how to deposit a denser and less defective coating. In our previous works [6–8] we used several analytical techniques for defect analysis. In this work, we present the results of coating topography investigation by means of 3D stylus profilometry. The field emission scanning electron microscope (SEM) was also used to study of the defect morphology. An SEM in combination with a focused ion beam technique (FIB) and atomic force microscopy (AFM) analyses were performed on selected samples. 2. Experimental
* Corresponding author. E-mail address: [email protected] (P. Panjan). 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.05.018
The BAI 730M (Balzers) deposition system with a thermionic arc was used for deposition of TiN and CrN, while the magnetron
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P. Panjan et al. / Vacuum 84 (2010) 209–214
sputtering system CC800/7 (CemeCon) was used for deposition of TiAlN, TiN, CrN and CrN/TiAlN multilayer hard coatings. Four types of substrates were used: a powder metallurgical high speed steel ASP30, a hot work tool steel H11, a cold work tool steel D2 and cemented carbide. The substrates were polished, ultrasonically cleaned and dried in hot air. Prior to deposition they were cleaned by ion etching. In the BAI 730 system, the thermionic arc in a triode arrangement is used for heating, etching and evaporation process steps. Prior to deposition, the substrates were heated to 450 C. They were then Ar ion sputter-cleaned for 15 min with an applied substrate bias of 200 V. During deposition the bias voltage was 125 V and the deposition rate of TiN and CrN hard coatings was about 50 nm/min. The CC800 system is equipped with four 8 kW unbalanced magnetron sources. During substrate cleaning an RF bias was applied (maximum RF power was 2 kW), while the etching time was 85 min. The substrate temperature was about 450 C. During deposition the bias voltage was 100 V. During deposition of TiAlN, TiN and CrN coatings the power on targets was 8 kW, 8 kW and 3 kW, respectively. During deposition the substrates undergo one-, two- or threeaxis planetary rotation. The surface morphology of the coatings was examined using a 3D stylus profilometer (Taylor Hobson Talysurf). Its vertical resolution was a few nm, while the lateral resolution was limited to 1 mm. The measured area was 1 mm 1 mm. A field emission scanning electron microscope (Zeiss Supra 35 VP) was used for examination of the morphology and microstructure of coating in planar surface view and cross-sectional fracture view. A focused ion beam (FIB) workstation was used to prepare a cross-section through the defects. We used FIB integrated in the FEI QUANTA 200 3D microscope. An ion beam was used to remove precise sections of material (close to the defect) from the specimen surface by sputtering. 3. Results and discussion
Fig. 1. Optical micrograph of the ball crater on the ASP30 substrate coated with a 8 mm thick TiN. Black dots on ground section are the cross sections of defects.
compressive stresses in deposit increases linearly with deposition time. At the beginning of the deposition process such stresses in the deposit were small while, after previous deposition, the critical deposit material was removed due to thermal stresses during cooling of the chamber. After a certain deposition time the compressive stresses in the deposit exceeded once again the critical values and the delamination process became more intense. 3.2. SEM and FIB study of defects Field emission SEM was used for examination of the morphology and microstructure of coatings in planar surface view and cross-sectional fracture view. Hard coating morphology can be treated on several levels:
3.1. Defect study by optical microscopy In the optical images the defects are seen as black dots. However, in the optical image the peaks (protrusions) and holes cannot be distinguished. In addition, lateral and depth resolution is not good enough for detailed analysis of defects. By comparison of optical images of the same area of substrate surface before and after deposition we can observe that some of the defects after deposition remain at the same position, some of them disappear, while some additional new defects appear. Based on this we can conclude that the origin of one part of growth defects arises from pretreatment of the substrate surface before deposition, while the second group of defects appears during deposition. Using appropriate software for image processing, it is possible to perform qualitative and quantitative analysis of defects. The optical microscopy was used also in combination with a ballcratering technique. The ball-cratering preparation technique was employed to prepare a low-angle cross-section of the coating. The concentration of defects on the ground section was observed by an optical microscope. Fig. 1 shows an optical micrograph of the ball crater on the ASP30 substrate coated with an 8 mm thick TiN (coating thickness t was determined by equation t ¼ (Y2–X2)/8R, where X and Y are inner and outer diameters of the circle and R the ball radius). It is evident that the concentration of defects is much higher close to the top surface while there are much fewer defects at the substratecoating interface. This means that most defects appeared after a certain deposition time. Our explanation is that the delamination of small particles from chamber components (shields, fixturing) is much more frequent with the deposition time, because the
- nanoscale: the morphology is determined by structural growth defects - submicroscale: the morphology is determined by nucleation mechanisms. Depending on growth conditions coarse columnar, fine columnar or amorphous growth modes are possible. The coating surface morphology is determined by the top morphology of columns. - microscale: morphological features are determined by growth defects (voids, flake, overcoated particles, pin-holes) - macroscale: the morphology is determined by the topography of bare substrate (scratches, pits, grooves, ridges). All morphological features of bare substrates are transferred and magnified through the coating. Thus on the SEM plan view of coatings we can clearly see the hillocks and pits which are at the position of carbide inclusions in the substrate. It is well known that hillocks and pits are generated by polishing and ion etching at the position of carbide inclusions. On Fig. 2a, which shows the surface morphology of TiAlN coating on powder metallurgical tool steel ASP30 substrate, a circular flat-topped area (approximately 1 mm in diameter) corresponding to overcoated carbides are visible. Similar features were observed on coated D2 tool steel (Fig. 2b). Their shape reflected the irregular form of chromium carbides in the steel matrix (typical dimension between 3 and 15 mm). We did not observe such morphological features on hot working tool steel H11 and cemented-carbide substrates. The other morphological features on the microscale are determined by growth defects. They can be classified as follows:
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Fig. 2. Plan view (a,b,c,d,h), cross-sectional SEM (e,g,i,j,k) and FIB images (f,l) of the following types of defects: (a) circular flat-topped morphological features at carbide inclusions in ASP30 tool steel (b) irregular flat-topped morphological features at carbide inclusions in D2 tool steel, (c) nodular or flake defect, (d) foreign particles preventing etching of the surface covered by them (e) cross-section of flake defect (f) FIB image of flake cross-section, (g,h) through-voids or dish-like craters (i,j) cone-like defects (k) SEM image of pin-hole fracture cross-section (l) FIB image of pin-hole cross-section.
(a) Defects in the form of flakes Nodular or flake defects are large (typical dimension is 5–40 mm) and irregularly shaped (Fig. 2c). They are loosely-bound defects, which form at asperities, debris and other foreign particles. Such particles come either from the vacuum chamber (heaters, shields and other components) or they stick to the substrate surface before the deposition process (metal dust, debris, polishing residues). Surface imperfections are preferred nucleation sites at which the coating is growing more rapidly than in the bulk deposit. Foreign particles also prevent etching of the substrate surface covered by them (Fig. 2d). The shadowing effect of such particles results in weak bonding between the overcoated particle and the surrounding coating. Therefore they can be easily detached due to the internal or thermal stresses. The coated particle could be also deformed. In this case a gap between the coating and the defect appears which can serve as crack initiators (Fig. 2e). Fig. 2f shows a FIB image in cross-section of the flake defect. In this case the foreign flake particle was built into the coating after a certain deposition time. EDX analysis revealed that its composition is iron-based.
(b) Open void defects The coated foreign particles are loosely bound to the coating and, during deposition, a part of them can spontaneously flake
off due to high compressive stress in the coating above critical thickness. Part of them flakes off during the cooling of the vacuum chamber due to thermal stress. This leads to the formation of through-voids (Fig. 2g). Actually voids can be interpreted as ‘‘inverse’’ flakes. If the flake is detached during deposition, the crater surface is covered by a thinner layer of hard coating with rough and porous features (Fig. 2h). (c) Cone-like defects These defects are smaller and have round, cone-like shape with a diameter of approximately 1 mm. They originate from fine particles (100–400 nm), which flake from the vacuum chamber components. On their way through plasma they can get charged. Such particles are transported to the substrate and they will be held to it by an electrostatic force, which is much stronger than gravity. During the deposition process they can be built into the coating forming cone-like defects. The flaking of such small particles can be caused by thermal and internal stresses in deposits on chamber components. As explained before, the number of such particles increases with deposition time. The defects which originate from such inclusions look like a cone in cross-section (Fig. 2i). Small foreign particles of submicrometer size which are built into a coating are difficult to identify from a cross-sectional fracture SEM image because the composition of dust defects is usually the same as composition of the coating,
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but they can be clearly visible in multilayer hard coatings. From the layer contour form we can find the positions at which such particles are built into the coating (Fig. 2j).
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(d) Pin-hole defects Defects, in the form of pin-holes extend through the whole coating (Fig. 2k, l). They are generated at the substrate pits (cavities) mostly formed during polishing [7]. Those cavities which are too narrow cannot be covered completely by a relatively thin hard coating due to a shadowing effect and, the coating is preferentially deposited to the flat front side of the substrate, while the deposition rate on the sidewall of the cavity is much smaller. The result is the formation of pin-holes. In smaller holes the opening is closed after a certain deposition time and a void is formed below it. Larger holes are never closed. Due to the shadowing effect, coating on the sidewalls of the hole has a columnar, porous structure.
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Growth defects of various sizes are typically non-uniformly distributed over the coating surface and their surface density is rather low (typically several tens up to a thousand defects per mm2). We found that the portion of the coating surface covered by defects is 0.5–4%. In order to obtain the correct defect density we have to analyze a relatively large surface area. For this purpose we used a 3D stylus profilometer, which has a very good lateral (0.25 mm in the xdirection, 1 mm in the y-direction) and vertical resolution (approximately 5 nm). Thus we obtained a 3D image (Fig. 3a,b) of the coating surface on a large scanning area with all the micrometer-sized details. The initial result of the 3D stylus profilometry is a map z(x,y) over the scanning range. Some corrections have to be applied to level the profile, exclude the general geometrical shape or possible measurement-induced misfits. Software for surface analysis usually gives several parameters regarding the peak (or hole) statistics: number of peaks, average peak height, average peak volume, average peak surface, total volume of peaks, maximum peak height, surface roughness, etc. A threshold value of what is to be counted as a peak or a hole is necessary for some of these parameters. We performed 3D profile measurements and detailed analysis on a series of different samples. Each sample was measured twice and each analysis was performed separately. The thresholds for peaks and craters were 0.5 mm and 0.25 mm, respectively. We selected a lower threshold for holes, because due to the conical shape of the stylus tip the depth of narrow holes can be underestimated. The stylus tip has a radius of curvature of 1.5–2.5 mm, and the cone angle is 90 . From the 3D image we estimated the surface density and height distribution of the defects. The influence of PVD deposition technique, deposition time, deposition rate, substrate type and its
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(e) Defects in the form of microdroplets Such types of defect are a consequence of arcing. All impurities on targets and substrates increase the arc tendency during etching or deposition steps. Arcs can cause the emission of microdroplets that are incorporated into a coating during its growth. Such defects are frequent especially in the case of cathodic arc deposition. We rarely find such kind of defects in hard coatings prepared by ion plating with thermoionic arc or by sputter deposition. However, we do not preclude the indirect influence of arcing on the formation of growth defects. Due to the high energy which arises during arcing at certain points of the substrate table, anode and other components of the vacuum chamber, we can expect that locally high thermal stresses appear. Such stresses could be one of the reasons for delamination of deposited coatings.
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Fig. 3. Typical 3D image (top view) of a TiAlN coating deposited on cold-working steel D2 and 3D image (3D view) of crater and peak on smaller scale (bottom).
location and orientation in the chamber on defect density was also analyzed. The result of 3D profile measurements are: - The reproducibility for the same sample is rather good. We observed that the number of peaks decreases by approximately 10% after the second measurements on the same surface area, while the number of craters increases for the same value. The reason could be the removal of some loosely bound coated particles by stylus tip and formation of craters. - The 3D profile image of the substrate surface after cleaning and after etching did not show any peaks, while the tip removes the particles which are on the substrate surface. Therefore it could not detect loosely-bound particles on bare substrates after cleaning and ion etching, although they were observed with an optical microscope. 3D profile images of such substrates show only a low density of craters (pits). It is well known that such defects appear already during mechanical pretreatment of tool steel substrates where some carbide inclusions could be torn out of material during polishing, leaving pits on the surface. - We found a correlation between the peak density and the crater density (Fig. 4a). The correlation is stronger for sputterdeposited coatings than for the evaporated ones. This can be explained by partial transformation of peaks into craters (part of coated foreign particles flake off due to the stresses). This process is more intensive for sputtered than for evaporated coatings due to the higher compressive stresses. - The number of peaks exceeds the number of craters by an order of magnitude (Fig. 4b). This means that only a smaller number of peaks (flakes) transformed into a craters. - Peaks may be several times higher than the film thickness, while the craters can be, at most, as deep as the film. We
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observed also that the higher is the defect density, the higher are the peaks and the deeper are the craters (Fig. 4c). There is also a good correlation between the defect density and surface roughness Sa. As was expected Sa increases with increasing defect density (Fig. 4d). A weak correlation, between the roughness and frequency of arcs on the substrate table was observed. A possible mechanism which is responsible for this correlation is explained in paragraph 3.2e. If we compare the samples of the same batch (the same deposition time) than there is no correlation between the coating thickness (it depends on substrate position and its rotation mode) and the defect density. This means that the same number of defects were built in the coatings irrespective of its thickness. The defect density depends on deposition time. The longer the times, the more defects are built into the coating. The density of defects depends on the position of the substrate in the deposition chamber. If the substrate is in a horizontal position (substrate surface is perpendicular to the target surface), then the defect density is two times larger on the top side of the substrate than on the bottom one. There is a clear trend of decreasing defect density for a sample in a vertical position (substrate surface is parallel to the on target surface). It is higher on the substrate at the bottom of the vacuum chamber, lower at top). Another parameter is the orientation of the sample. The vertically-mounted samples have a lower defect density than the horizontally mounted ones. The same coatings deposited on different substrate materials are not distinguished by difference in defect density. However, we found that the increase of surface roughness is different for different substrates. This could be explained by different ion etching effects. Different tool steels are composed of carbides
of various sizes and composition embedded in the matrix. The sputtering rates of these carbides and the matrix vary. This results in formation of a large number of pits and hillocks, while the surface roughness increases. - Different hard coatings prepared by the same deposition technique are distinguished by the number of defects. We found the lowest number of defects in sputter-deposited CrN coatings, more in TiAlN, while most defects are found, in the TiN coating. - Coatings prepared by different deposition techniques are distinguished by the defect density. The coatings deposited by thermoionic ion plating are, in general, smoother in comparison to the sputter-deposited coatings.
4. Conclusions We used 3D profilometry to estimate the surface density and height distribution of the defects. We found that defect density depends on deposition time, deposition technique, deposition parameters, substrate position in the vacuum chamber, its orientation and rotation mode. We did not find any correlation of defect density with coating thickness and substrate material, while the correlation of defect density with arcing was not very pronounced. We have also shown that SEM in combination with FIB provide a very useful technique for structural and compositional analysis of selected growth defects.
Acknowledgements This work was supported by the Slovenian Research Agency (project L2-9189).
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