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Correlation between the interface structure of a TiN coating deposited onto AISI 304 and the coating adhesion
Surface and Coatings Technology, 61 (1993) 227—232
227
Correlation between the interface structure of a TiN coating deposited onto AISI 304 and the coating adhesion C. Quaeyhaegensa, J.
D~Haena,L. M. Stalsa, M. Van Stappenb, F. Bodartc and G. Terwagnec
~Institutefor Materials Research, Materials Physics Division, Science Park, Universitaire Campus, B-3590 Diepenbeek (Belgium) bScientific and Technical Centrefor the Metalworking Industry, WTCM, Science Park, Universitaire Campus, B-3590 Diepenbeek (Belgium) °FacultésUniversitaires Notre-Dame de Ia Paix, Laboratoire d’Analyses par Réctions Nucléires, 22 rue Muzet, B-5000 Namur (Belgium)
Abstract The interface between austenitic stainless steel AISI 304 and three different types of TiN coating, one with and two without an intermediate Ti layer, deposited using the triode evaporation ion plating technique, were characterized using cross-sectional transmission electron microscopy, glancing angle X-ray diffraction and resonant nuclear reaction analysis. A highly faulted interface due to the initial plasma nitriding of the substrates is observed for one type of deposition without the intermediate Ti layer. In this case a phase is formed at the interface between AISI 304 and the TiN coating, identified as expanded austenite. Scratch tests show poor coating adhesion of these samples. In contrast, a well adherent TiN coating can be deposited onto austenitic stainless steel without an intermediate Ti layer when the substrate material is not plasma nitrided and a defect-free interface is formed.
1. Introduction Hard wear-resistant TiN coatings on tools can only be introduced successfully for applications in the metal working industry in those cases where good coating adhesion to the substrate material can be obtained. It is generally known that pretreatment of the surface of metallic samples prior to coating is essential for optimization of the coating adhesion. For TiN it is observed that better adhesion can be obtained when an intermediate Ti layer is deposited prior to the TiN coating. Rickerby and Newbery [1] determined the optimal thickness of the intermediate Ti layer to be about 200 nm. Cheng et al. [2] studied TiN coatings deposited using the evaporation ion plating technique on M 50 tool steel with and without an intermediate Ti layer and they observed direct spalling and buckling-up of the coating when no intermediate layer was deposited. These results may indicate that the deposition of an intermediate Ti layer is a necessary condition to obtain a well adherent TiN coating on tool steel. However, a well adherent TiN coating without an intermediate Ti layer has been obtained using magnetron sputtering techniques [1, 3]. In the literature, different hypotheses have been formulated to explain why an intermediate Ti layer is beneficial for adhesion of the TiN coating [2—4].In recent publications Quaeyhaegens and coworkers [5—7] showed that when no intermediate Ti layer is deposited between a steel substrate and a TiN coating, and the deposition conditions are such that the metallic substrate is exposed
directly to the nitrogen atoms and ion flux, good adhesion cannot be obtained. In these publications [5,6] the interface phase formed between an austenitic stainless steel substrate and the TiN coating was described as y’Fe4N. However, there is some doubt about this phase identification [8]. Therefore, in this work the interface between austenitic stainless steel and a TiN coating was studied in more detail with different analytical techniques for different types of deposition. A correlation is made between the analytical results and the coating adhesion.
2. Experimental details 2.1. Deposition conditions The substrate material used was austenitic stainless steel type AISI 304 (0.03% C, 0.75%—l% Si, 2% Mn, 18%—20% Cr, 8%—12% Ni). Three types of TiN deposition experiments were performed in a Balzers BAK 640 evaporation ion plating installation. The deposition temperature was between 375 °Cand 425 °Cfor all experiments. The specimen pretreatment and the different deposition experiments are described in detail in ref. 6. For the sake of clearness, the different deposition processes are summarized in Table 1. Deposition type 1 is the conventional deposition of a TiN coating with an intermediate Ti layer (standard process). The thickness of the intermediate Ti layer is 130 ±25 nm. The difference between type 2 and type 3 depositions is that in the type 2 deposition the substrates are masked from the plasma during the initial stage of the deposition
228
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Interface structure and adhesion
TABLE I. Summary of deposition processes Type of deposition
Coating system
Remarks
2
Ti—TiN TiN
3
TiN
Standard coating process Substrates masked from plasma during initial stage of deposition Substrates not masked from plasma during initial stage of deposition
process. After stabilization of both Ti evaporation and the nitrogen gas flow the samples are exposed to the plasma. In contrast, in the type 3 deposition process the samples are exposed to an unavoidable non-stationary Ti evaporation rate and nitrogen gas flow during the starting-up phase of the Ti evaporation. Therefore, in type 3 experiments, plasma nitriding of the substrate material during the initial stage of the deposition process is possible. The three deposition processes were used to produce samples with two TiN coating thicknesses: (170 ±20) nm and (1460 ±170) nm. 2.2. Analysis ofcoatings and interfaces The adhesion of the coatings on the substrates was evaluated using the scratch test method (CSEM Revetest). Scratch tests were performed on both 170 nm and 1460 nm thick TiN coatings with a translation speed of the table of 10 mm min 1 and a rate of load increase of 100 N mm 1 The scratches were examined by optical reflection microscopy (Reichert MeF2). Glancing angle X-ray diffraction (GXRD) was carried out using a Philips diffractometer with Cr Kx radiation [6]. Because diffractograms are recorded at low incidence angles of the primary X-ray beam, a correction factor for the diffraction peak position has to be taken into account (see for example ref. 9). Cross-sectional transmission electron microscopy (XTEM) was performed in a Philips CM12 microscope, operating at 120 kV and equipped with an EDAX energy dispersive analyser. The crosssectional samples of the coated substrates were prepared by a technique similar to that described by Helmersson and Sundgren [10] using a Gatan dimple grinder and a BAL-TEC ion milling apparatus, the latter operating at a voltage of 5 kV. During the Ar etching process two shields were placed which allow ion beam milling in two 90°sectors perpendicular to the interface. The angle at which the Ar ions bombard the surface is changed from 150 to 11°as the milling operation proceeds. Resonant nuclear reaction analysis (RNRA) was used to determine the nitrogen concentration [11] in the samples using the resonance of the proton induced nuclear reaction ‘5N(p,c~y)12Cat a proton energy E~of 429 keY (resonance width of 120 eY). The protons are accelerated in a van de Graaff accelerator. Depth profiling was per-
formed by increasing the proton beam energy starting from 429 keV in steps of 2.7 keY. Because GXRD and RNRA are typically used for interface studies, they were only carried out on samples with the lower TiN coating thickness of 170 nm. XTEM was carried out on the 1460 nm thick TiN coating. 3. Results 3.!. Adhesion of the TiN coating Figures 1(a)—1(c) are schematic representations, based on inspection by optical reflection microscopy, of the different scratch profiles obtained for the different specimen types for a TiN coating thickness of 1460 nm. The shaded zones correspond to areas where the coating is removed from the substrate. As shown in Fig. 1, in addition to semicircular cracks in front of the indenter, cracks in the TiN coating outside the scratches can also be clearly observed. Optimal analytical characterization of the interface between the coating and substrate material with GXRD and RNRA can only be performed for samples with a much thinner TiN coating than the 1460 nm thick coating on which the scratch tests shown in Fig. I were carried out. Therefore scratch tests were also performed for the three types of samples with a TiN coating thickness of 170 nm. Figures 2(a)—2(c) are schematic representations of the different scratch profiles obtained for these samples. The shaded zones correspond again to areas where the coating is removed and where the substrate material can be observed. Furthermore, cracks in the TiN coating outside the scratches can also
0
20
40 (NEWTON)
60
80
20
40 (NEWTON)
60
80
40
60
80
b
0
c
20
(NEWTON)
Fig. of the scratch patterns obtained for 1460 I.nmSchematic thick TiNrepresentation coatings: (a) type I deposition with a 130 ±25 nm thick intermediate Ti layer; (b) type 2 deposition and (c) type 3 deposition.
/
C. Quaeyhaegens et a!. o
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-
O
_‘~ ‘
~
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.‘ ..,‘ - ....~....
~
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—~
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- ~
.t-
~
40
60
~
/)~ 80
(NEWTON)
c
0
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40 (NEWTON)
60
80
Fig. 2. Schematic representation of the scratch patterns obtained for 170 nm thick TiN coatings: (a) type 1 deposition with a 130 ±25 nm thick intermediate Ti layer; (b) type 2 deposition and (c) type 3 deposition.
be observed. However, no semicircular cracks in the scratches are observed in this case. 3.2. Analytical characterization 3.2.1. Type I deposition Figure 3 is an XTEM image of the three-layer system AISI 304—Ti—TiN with the TiN layer at the top, austenite grains at the bottom, and in between these two layers an intermediate Ti layer which is about lOOnm thick, The intermediate layer contains unidentified grains,
o~
_
-
__~4~~
Indeed the grain indicated with an arrow forms a convergent diffraction shownf.c.c. in the inset tobeam Fig. 3electron corresponding to pattern a B=[0ll] crystal direction vector. The spots of this convergent beam electron diffraction pattern correspond to an f.c.c. structure with d-values of about 0.251 nm for the (Ill) planes and about 0.218 nm for the (200) planes. The
80
(NEWTON)
b
229
Interface structure and adhesion
observation of an f.c.c. structure instead of the h.c.p. structure for TiN indicates that the intermediate Ti layer is not pure Ti. The observed lattice plane distances could be in agreement with those for TiC or TiO, although another unidentified phase could also be present. 3.2.2. Type 2 deposition The XTEM image in Fig. 4 shows the interface structure of AISI 304 on which TiN has been deposited in a type 2 experiment. The lower side of the figure reveals long and small grains of the substrate lying under the interface which runs through points a and b on the micrograph. On the upper side a large TiN grain grows over several austenite grains beneath the grain boundary. It should be noticed that the interface and the TiN coating show only a few defects. 3.2.3. Type 3 deposition Figure 5 shows the result of GXRD measurements for an AISI 304 substrate on which a TiN coating was deposited in a type 3 experiment. Four different incidence angles O~ were used: 0.750 ±0.050, 1.00°±0.05°, 1.500 ±0.050 and 2.500 ±0.050. After deconvolution of the spectrum diffraction peaks of the (200) and (311) lattice plane reflections of TiN and the (111), (200) and (220) lattice plane reflections of the austenitic substrate material (labelled y-Fe) can be identified. Furthermore, additional diffraction peaks (labelled UI.) can clearly be observed. From Fig. 5 it is clear that when the incidence angle of the primary X-ray beam is decreased the relative intensity of the diffraction peaks of the U.I. phase increases. When the incidence angle is 0.75°±0.05°only the (200) lattice plane reflection of TiN can be observed. From these observations it is clear that an interface phase is formed between the bulk of the substrate material and the TiN coating. However, an unambiguous phase identification cannot be made on the basis of
__________
.~
Fig. 3. XTEM image of the interface of a type I sample: the inset is an electron diffraction pattern of an unidentified f.c.c. grain in the intermediate Ti layer (indicated by an arrow).
__
___________
__
__________
Fig. 4. XTEM image of the interface of a iype 2 sample.
230
C. Quaeyhaegens et al.
100.
/
Interface structure and adhesion
of variable size (from several nanometres in the neighbourhood of the interface to about 50 nm at the outer surface of the coating). At the substrate side of the interface, a layer with long, heavily faulted grains of about 50 nm diameter and 25 nm thickness can be observed. The left-hand inset to Fig. convergent electron diffraction pattern of 6theshows grainthe marked
—F (~i)
7—Fe
y—Fe
(200)
u. ~
a in the TiN layer. The right-hand inset to Fig. 6 shows a B = [001] cubic structure of the faulted grain marked b. It is clear that the form of these grains is very different
(220)
TIN
(200 _________
64~
TiN
~
~
68 76~
20
~
80~122~
13T
Fig. 5. GXRD measurement for type 3 deposition with 01 0.75°±0.05°(a), 01 = 1.00°±0.05°(b), 0~= 1.50°±0.05°(c) and 01 2.50°±0.05°(d).
= =
GXRD measurements alone. Although no superlattice plane reflections are observed pointing to a y’-Fe4N structure, both expanded austenite and y’-Fe4N can be attributed to the unidentified lattice plane reflections in Fig. 5. Figure 6 shows an XTEM image of the interface between the austenitic substrate (lower part) and the TiN coating (upper part) for a type 3 deposition. The interface runs from the left arrow to the right arrow on the micrograph. The interface contains many defects such as voids and cracks which can also be observed in the initial TiN layer. The TiN coating consists of grains
for respectively the (111), (200) and (220) lattice plane reflections. These values could be of austenite but, owing to the different shape of these grains compared with the austenite grains in Fig. 4, and taking into account the result of the GXRD characterization, this phase can be identified as either expanded austenite or ‘/-Fe4N. Although no supperlattice plane reflections are observed in the electron diffraction pattern which should point to a y’-Fe4N structure, unambiguous phase identification is not possible based on the GXRD and XTEM study only. In Fig. 7 the RNRA depth profile obtained on a sample coated according to a type 3 deposition is shown. From this figure it is clear that the atomic nitrogen concentration in the TiN coating is about 44 ±3 at.%. At a depth of 0.2 ~.tmthe atomic nitrogen concentration drops to 12.5 ±3 at.%. Because the coating thickness is estimated to be 170 ±20 nm, the latter nitrogen concentration is measured in the substrate material.
4.
__________
______
(type 2 deposition). The heavily grains cubic structure have d-values of 0.211 faulted nm, 0.183 nm,with 0.129 nm
I
_____
~
intrinsic test parameters (translational
speed of the table and load increase) of the scratch tests
z z 0 I—
~
Fig. 6. XTEM image of the interface of a type 3 sample: the insets a and b are electron diffraction patterns of the grains indicated by and b respectively on the micrograph. The interface runs from the lefthand arrow towards the right-hand arrow.
o~0
0.2
~
0
~
0.4
o
0.6
DEPTH (j.a~i)
Fig. 7. RNRA depth profile for a type 3 sample.
00
~5at%)
0.8
C. Quaeyhaegens et a!.
/
Interface structure and adhesion
231
are the same for all the samples tested, a direct comparison can be made between the scratches observed for the different types of TiN depositions when the TiN coating
observed with XTEM (Fig. 6) at the interface between the TiN coating and the bulk of the austenitic substrate. These grains are either expanded austenite or ~“-Fe4N.
thickness is the same. From Fig. 1 it is clear that the adhesion of a 1460 nm thick TiN coating is different for the three deposition types. Indeed for a type 3 deposition the coating is removed from the substrate already at a very low load (less than 5 N). In contrast, for types 1 and 2 deposition the coating is well adherent although some differences can be observed. The most important difference is the appearance of semicircular cracks in front of the indenter at lower loads for type 2 deposition than for type 1 deposition. These semicircular cracks are caused by bending of the TiN coating in front of the moving stylus [12]. The results obtained for the three types of deposition are in good agreement with those reported earlier [6]. From Fig. 2 it follows that a similar general trend is observed for the 170 nm thick coating, except that owing to the lower coating thickness no semicircular cracks are observed in the scratches. The XTEM study shows clear differences in interface structure for the different types of deposition. In a type 1 deposition an intermediate Ti layer can be observed which is not pure Ti. The presumable presence of TiO grains, which is probably due to deoxidation of the substrate surface, can thus promote coating adhesion [4]. A type 2 deposition shows a defect-free interface between the TiN coating and AISI 304. In contrast to the type 2 deposition, for type 3 deposition a heavily faulted interface and defects in the initial TiN coating can be observed which result from the initial plasma nitriding of the substrate material. The poor coating adhesion observed for type 3 deposition can be correlated with the formation of this highly defected interface. The unidentified lattice plane reflections observed with GXRD (Fig. 5) are ascribed to the heavily faulted grains
Although the crystallographic difference between expanded austenite and y’-Fe4N [13, 14] is well established, it remains difficult to distinguish between them. Indeed in both phases the nitrogen atoms are in interstitial octahedral positions but the difference between the phases is the positioning of the nitrogen atoms in the crystallographic lattice. These atoms are ordered in y’-Fe4N and are randomly distributed in expanded austenite. This distribution leads to a primitive cubic lattice for the former phase and to an f.c.c. lattice for the latter phase. Although both phases have a different crystallographic structure it is not possible to distinguish between them using an electron diffraction pattern if no superlattice plane electron diffraction spots are clearly observable. It is however possible, using TEM characterization with the convergent beam electron diffraction (CBED) technique, to distinguish between expanded austenite and y’-Fe4N if the phases are not disturbed. Indeed expanded austenite belongs to point group Fm3m and y’-Fe4N belongs to point group P43m. These point groups correspond to different CBED patterns for a specific zone axis. As a consequence, CBED patterns can be used for phase identification. However, the heavily faulted interface grains in this study disturb the symmetry of the CBED patterns which complicates the distinction between the phases. If an equilibrium Fe—N phase is formed at the interface then determination of the atomic nitrogen concentration can be used to distinguish between the phases. Indeed if y’-Fe4N is formed, an atomic nitrogen concentration of 20.00 ±0.15 at.% must be measured at the interface. From Fig. 7 it is clear that the atomic nitrogen concentration at the interface is only 12.5 ±3 at.%. Therefore
20ob~’
A20, 20caic, dhk, and a
TABLE 2.
0 for the lattice plane reflections in Fig. 5 (0~= 1.5°±0.05~) ~ (deg)
A20 (deg)
2Ocaic (deg)
dhkf (nm)
a0 (nm)
67.08 ±0.03 79.20 ±0.03 128.75 ±0.03
0.12 0.12 0.12
66.96 ±0.03 79.08 ±0.03 128.63 ±0.03
0.2077 ±0.0001 0.17994 ±0.00006 0.12711 ±0.00002
0.3598 ±0.0002 0.3599 ±0.0001 0.35952 ±0.00006
Expanded austenite (III) 66.04 ±0.05 (200) 78.08 ±0.04 (220) 125.64 ±0.10
0.12 0.12 0.12
65.92 ±0.05 77.96 ±0.04 125.52 ±0.10
0.2105 ±0.0001 0.1821 ±0.0001 0.12884 ±0.00006
0.3647 ±0.0003 0.3642 ±0.0002 0.3643 ±0.0001
TiN (111) (200) (220) (311) (222)
0.08 0.08 0.08 0.08 0.09
55.35 ±0.03 65.21 ±0.10 99.77 ±0.03 128.57±0.10 142.18 ±0.05
0.2466 ±0.0001 0.2126 ±0.0003 0.14978 ±0.00003 0.1271 ±0.0002 0.1211 ±0.0002
0.4271 ±0.0002 0.4252 ±0.0006 0.42364 ±0.00008 0.4215±0.0008 0.4195 ±0.0007
AISI 304 (111) (200) (220)
55.43 ±0.03 65.29 ±0.10 99.85 ±0.03 128.65±0.10 142.27 ±0.05
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Interface structure and adhesion
the unidentified phase in Fig. 5 is attributed to expanded austenite. In Table 2 the lattice plane distances are given after deconvolution of the observed lattice plane reflections 20obs of Fig. 5, for AISI 304, expanded austenite and TiN. The lattice plane distances dhkj are calculated using the corrected 28 values 20caic~ The latter were determined as explained in ref. 9. In Table 2 the correction factor (A20 = 20caic) is also given. From Table 2 it is clear that different lattice parameters a 0 are obtained for the different lattice planes. The differences in a0 can probably be attributed to strains present in the TiN coating and the austenitic substrate. —
Kernwetenschappen, for financial support under Contract 4.0004.91. Financial support of the Flemish Region, the Wallone Region, the Brussels Capital Region and the Belgian Science Supporting Institute, Instituut ter Bevordering van het Wetenschappelijk Onderzoek in Nijverheid en Landbouw, under Contract CI 1/47674/080 is acknowledged. This text presents research results of the Belgian programme on Interuniversity poles of attraction initiated by the Belgian State, Prime Minister’s Office—Science Policy Programming. Scientific responsibility is assumed by the authors.
References 5. Conclusions 1 D. S. Rickerby and R. B. Newbery, Proc. 6th
By means of glancing angle X-ray diffraction and cross-sectional transmission electron microscopy it was possible to reveal different interface structures for different types of TiN deposition. The poor coating adhesion of TiN on steel substrates generally observed when no intermediate Ti layer is deposited could be attributed to the formation of a highly faulted interface due to the initial plasma nitriding of the substrates. The phase formed at the interface between AISI 304 and TiN due to plasma nitriding of the substrate could be identified as expanded austenite on the basis of measurements of the nitrogen concentration using resonant nuclear reaction analysis. In contrast to what is generally expected, a well adherent TiN coating can be deposited onto austenitic stainless steel without an intermediate Ti layer when the substrate material is not plasma nitrided and a defect-free interface is formed.
Acknowledgments Thanks are due to the Belgian Science Supporting Institute, Interuniversitair Instituut voor
2 3 4 5 6
mt.
Conf on Ion and
Plasma Assisted Techniques, Brighton, 1987, CEP Consultants, Edinburgh, 1987, p. 224. C. C. Cheng, A. Erdemir and G. R. Fenske, Surf. Coat. Technol., 39—40 (1989) 365. U. Helmersson, B. 0. Johansson, J. E. Sundgren, H. T. G. Hentzell and P. Billgren, J. Vac. Sci. Technol. A, 3(2) (1985) 308. M. Van Stappen, B. Malliet, L. De Schepper, L. M. Stals, J. P. Celis and J. R. Roos, Surf. Eng., 4 (1989) 305. C. Quaeyhaegens, L. M. Stals, M. Van Stappen and L. De Schepper, Thin Solid Films, 197 (1991) 37. C. Quaeyhaegens, L. M. Stals, L. De Schepper, M. Van Stappen
and B. Malliet, Surf Coat. Technol., 45 (1991) 193. 7 C. Quaeyhaegens, J. D’Haen, M. Van Stappen, L. De Schepper and L. M. Stals, Proc. 8th Int. Conf. on Ion and Plasma Assisted Techniques, Brussels, 1991, CEP Consultants, Edinburgh, 1991, ~. 296. 8 J. D’Haen, L. De Schepper, M. Van Stappen and L. M. Stals, Micron Microsc. Acta, 21(4) (1990) 279—280. 9 G. Lim, W. Parrish, C. Ortiz, M. Bellotto and M. Hart, J. Mater. Res., 2(4) (1987) 471. 10 U. Helmersson and J. E. Sundgren, J. Electron Microsc. Tech., 4 (1986) 361. 11 C. Quaeyhaegens, M. Van Stappen, L. M. Stals, F. Bodart, G. Terwagne and R. Vlaeminck, Surf Coat. Technol, 54—55 (1992) 279. 12 P. j. Burnett and D. S. Rickerby, Thin Solid Films, 154 (1987) 403. 13 K. H. Jack, Proc. R. Soc. London, Ser, A, 195 (1948) 34. 14 K. H. Jack, Proc. R. Soc. London, Ser, A, 208 (1951) 200.
227
Correlation between the interface structure of a TiN coating deposited onto AISI 304 and the coating adhesion C. Quaeyhaegensa, J.
D~Haena,L. M. Stalsa, M. Van Stappenb, F. Bodartc and G. Terwagnec
~Institutefor Materials Research, Materials Physics Division, Science Park, Universitaire Campus, B-3590 Diepenbeek (Belgium) bScientific and Technical Centrefor the Metalworking Industry, WTCM, Science Park, Universitaire Campus, B-3590 Diepenbeek (Belgium) °FacultésUniversitaires Notre-Dame de Ia Paix, Laboratoire d’Analyses par Réctions Nucléires, 22 rue Muzet, B-5000 Namur (Belgium)
Abstract The interface between austenitic stainless steel AISI 304 and three different types of TiN coating, one with and two without an intermediate Ti layer, deposited using the triode evaporation ion plating technique, were characterized using cross-sectional transmission electron microscopy, glancing angle X-ray diffraction and resonant nuclear reaction analysis. A highly faulted interface due to the initial plasma nitriding of the substrates is observed for one type of deposition without the intermediate Ti layer. In this case a phase is formed at the interface between AISI 304 and the TiN coating, identified as expanded austenite. Scratch tests show poor coating adhesion of these samples. In contrast, a well adherent TiN coating can be deposited onto austenitic stainless steel without an intermediate Ti layer when the substrate material is not plasma nitrided and a defect-free interface is formed.
1. Introduction Hard wear-resistant TiN coatings on tools can only be introduced successfully for applications in the metal working industry in those cases where good coating adhesion to the substrate material can be obtained. It is generally known that pretreatment of the surface of metallic samples prior to coating is essential for optimization of the coating adhesion. For TiN it is observed that better adhesion can be obtained when an intermediate Ti layer is deposited prior to the TiN coating. Rickerby and Newbery [1] determined the optimal thickness of the intermediate Ti layer to be about 200 nm. Cheng et al. [2] studied TiN coatings deposited using the evaporation ion plating technique on M 50 tool steel with and without an intermediate Ti layer and they observed direct spalling and buckling-up of the coating when no intermediate layer was deposited. These results may indicate that the deposition of an intermediate Ti layer is a necessary condition to obtain a well adherent TiN coating on tool steel. However, a well adherent TiN coating without an intermediate Ti layer has been obtained using magnetron sputtering techniques [1, 3]. In the literature, different hypotheses have been formulated to explain why an intermediate Ti layer is beneficial for adhesion of the TiN coating [2—4].In recent publications Quaeyhaegens and coworkers [5—7] showed that when no intermediate Ti layer is deposited between a steel substrate and a TiN coating, and the deposition conditions are such that the metallic substrate is exposed
directly to the nitrogen atoms and ion flux, good adhesion cannot be obtained. In these publications [5,6] the interface phase formed between an austenitic stainless steel substrate and the TiN coating was described as y’Fe4N. However, there is some doubt about this phase identification [8]. Therefore, in this work the interface between austenitic stainless steel and a TiN coating was studied in more detail with different analytical techniques for different types of deposition. A correlation is made between the analytical results and the coating adhesion.
2. Experimental details 2.1. Deposition conditions The substrate material used was austenitic stainless steel type AISI 304 (0.03% C, 0.75%—l% Si, 2% Mn, 18%—20% Cr, 8%—12% Ni). Three types of TiN deposition experiments were performed in a Balzers BAK 640 evaporation ion plating installation. The deposition temperature was between 375 °Cand 425 °Cfor all experiments. The specimen pretreatment and the different deposition experiments are described in detail in ref. 6. For the sake of clearness, the different deposition processes are summarized in Table 1. Deposition type 1 is the conventional deposition of a TiN coating with an intermediate Ti layer (standard process). The thickness of the intermediate Ti layer is 130 ±25 nm. The difference between type 2 and type 3 depositions is that in the type 2 deposition the substrates are masked from the plasma during the initial stage of the deposition
228
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Interface structure and adhesion
TABLE I. Summary of deposition processes Type of deposition
Coating system
Remarks
2
Ti—TiN TiN
3
TiN
Standard coating process Substrates masked from plasma during initial stage of deposition Substrates not masked from plasma during initial stage of deposition
process. After stabilization of both Ti evaporation and the nitrogen gas flow the samples are exposed to the plasma. In contrast, in the type 3 deposition process the samples are exposed to an unavoidable non-stationary Ti evaporation rate and nitrogen gas flow during the starting-up phase of the Ti evaporation. Therefore, in type 3 experiments, plasma nitriding of the substrate material during the initial stage of the deposition process is possible. The three deposition processes were used to produce samples with two TiN coating thicknesses: (170 ±20) nm and (1460 ±170) nm. 2.2. Analysis ofcoatings and interfaces The adhesion of the coatings on the substrates was evaluated using the scratch test method (CSEM Revetest). Scratch tests were performed on both 170 nm and 1460 nm thick TiN coatings with a translation speed of the table of 10 mm min 1 and a rate of load increase of 100 N mm 1 The scratches were examined by optical reflection microscopy (Reichert MeF2). Glancing angle X-ray diffraction (GXRD) was carried out using a Philips diffractometer with Cr Kx radiation [6]. Because diffractograms are recorded at low incidence angles of the primary X-ray beam, a correction factor for the diffraction peak position has to be taken into account (see for example ref. 9). Cross-sectional transmission electron microscopy (XTEM) was performed in a Philips CM12 microscope, operating at 120 kV and equipped with an EDAX energy dispersive analyser. The crosssectional samples of the coated substrates were prepared by a technique similar to that described by Helmersson and Sundgren [10] using a Gatan dimple grinder and a BAL-TEC ion milling apparatus, the latter operating at a voltage of 5 kV. During the Ar etching process two shields were placed which allow ion beam milling in two 90°sectors perpendicular to the interface. The angle at which the Ar ions bombard the surface is changed from 150 to 11°as the milling operation proceeds. Resonant nuclear reaction analysis (RNRA) was used to determine the nitrogen concentration [11] in the samples using the resonance of the proton induced nuclear reaction ‘5N(p,c~y)12Cat a proton energy E~of 429 keY (resonance width of 120 eY). The protons are accelerated in a van de Graaff accelerator. Depth profiling was per-
formed by increasing the proton beam energy starting from 429 keV in steps of 2.7 keY. Because GXRD and RNRA are typically used for interface studies, they were only carried out on samples with the lower TiN coating thickness of 170 nm. XTEM was carried out on the 1460 nm thick TiN coating. 3. Results 3.!. Adhesion of the TiN coating Figures 1(a)—1(c) are schematic representations, based on inspection by optical reflection microscopy, of the different scratch profiles obtained for the different specimen types for a TiN coating thickness of 1460 nm. The shaded zones correspond to areas where the coating is removed from the substrate. As shown in Fig. 1, in addition to semicircular cracks in front of the indenter, cracks in the TiN coating outside the scratches can also be clearly observed. Optimal analytical characterization of the interface between the coating and substrate material with GXRD and RNRA can only be performed for samples with a much thinner TiN coating than the 1460 nm thick coating on which the scratch tests shown in Fig. I were carried out. Therefore scratch tests were also performed for the three types of samples with a TiN coating thickness of 170 nm. Figures 2(a)—2(c) are schematic representations of the different scratch profiles obtained for these samples. The shaded zones correspond again to areas where the coating is removed and where the substrate material can be observed. Furthermore, cracks in the TiN coating outside the scratches can also
0
20
40 (NEWTON)
60
80
20
40 (NEWTON)
60
80
40
60
80
b
0
c
20
(NEWTON)
Fig. of the scratch patterns obtained for 1460 I.nmSchematic thick TiNrepresentation coatings: (a) type I deposition with a 130 ±25 nm thick intermediate Ti layer; (b) type 2 deposition and (c) type 3 deposition.
/
C. Quaeyhaegens et a!. o
~ -
-
O
_‘~ ‘
~
~
.‘ ..,‘ - ....~....
~
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~
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)
~ -
—~
0
20
- ~
.t-
~
40
60
~
/)~ 80
(NEWTON)
c
0
20
40 (NEWTON)
60
80
Fig. 2. Schematic representation of the scratch patterns obtained for 170 nm thick TiN coatings: (a) type 1 deposition with a 130 ±25 nm thick intermediate Ti layer; (b) type 2 deposition and (c) type 3 deposition.
be observed. However, no semicircular cracks in the scratches are observed in this case. 3.2. Analytical characterization 3.2.1. Type I deposition Figure 3 is an XTEM image of the three-layer system AISI 304—Ti—TiN with the TiN layer at the top, austenite grains at the bottom, and in between these two layers an intermediate Ti layer which is about lOOnm thick, The intermediate layer contains unidentified grains,
o~
_
-
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Indeed the grain indicated with an arrow forms a convergent diffraction shownf.c.c. in the inset tobeam Fig. 3electron corresponding to pattern a B=[0ll] crystal direction vector. The spots of this convergent beam electron diffraction pattern correspond to an f.c.c. structure with d-values of about 0.251 nm for the (Ill) planes and about 0.218 nm for the (200) planes. The
80
(NEWTON)
b
229
Interface structure and adhesion
observation of an f.c.c. structure instead of the h.c.p. structure for TiN indicates that the intermediate Ti layer is not pure Ti. The observed lattice plane distances could be in agreement with those for TiC or TiO, although another unidentified phase could also be present. 3.2.2. Type 2 deposition The XTEM image in Fig. 4 shows the interface structure of AISI 304 on which TiN has been deposited in a type 2 experiment. The lower side of the figure reveals long and small grains of the substrate lying under the interface which runs through points a and b on the micrograph. On the upper side a large TiN grain grows over several austenite grains beneath the grain boundary. It should be noticed that the interface and the TiN coating show only a few defects. 3.2.3. Type 3 deposition Figure 5 shows the result of GXRD measurements for an AISI 304 substrate on which a TiN coating was deposited in a type 3 experiment. Four different incidence angles O~ were used: 0.750 ±0.050, 1.00°±0.05°, 1.500 ±0.050 and 2.500 ±0.050. After deconvolution of the spectrum diffraction peaks of the (200) and (311) lattice plane reflections of TiN and the (111), (200) and (220) lattice plane reflections of the austenitic substrate material (labelled y-Fe) can be identified. Furthermore, additional diffraction peaks (labelled UI.) can clearly be observed. From Fig. 5 it is clear that when the incidence angle of the primary X-ray beam is decreased the relative intensity of the diffraction peaks of the U.I. phase increases. When the incidence angle is 0.75°±0.05°only the (200) lattice plane reflection of TiN can be observed. From these observations it is clear that an interface phase is formed between the bulk of the substrate material and the TiN coating. However, an unambiguous phase identification cannot be made on the basis of
__________
.~
Fig. 3. XTEM image of the interface of a type I sample: the inset is an electron diffraction pattern of an unidentified f.c.c. grain in the intermediate Ti layer (indicated by an arrow).
__
___________
__
__________
Fig. 4. XTEM image of the interface of a iype 2 sample.
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C. Quaeyhaegens et al.
100.
/
Interface structure and adhesion
of variable size (from several nanometres in the neighbourhood of the interface to about 50 nm at the outer surface of the coating). At the substrate side of the interface, a layer with long, heavily faulted grains of about 50 nm diameter and 25 nm thickness can be observed. The left-hand inset to Fig. convergent electron diffraction pattern of 6theshows grainthe marked
—F (~i)
7—Fe
y—Fe
(200)
u. ~
a in the TiN layer. The right-hand inset to Fig. 6 shows a B = [001] cubic structure of the faulted grain marked b. It is clear that the form of these grains is very different
(220)
TIN
(200 _________
64~
TiN
~
~
68 76~
20
~
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13T
Fig. 5. GXRD measurement for type 3 deposition with 01 0.75°±0.05°(a), 01 = 1.00°±0.05°(b), 0~= 1.50°±0.05°(c) and 01 2.50°±0.05°(d).
= =
GXRD measurements alone. Although no superlattice plane reflections are observed pointing to a y’-Fe4N structure, both expanded austenite and y’-Fe4N can be attributed to the unidentified lattice plane reflections in Fig. 5. Figure 6 shows an XTEM image of the interface between the austenitic substrate (lower part) and the TiN coating (upper part) for a type 3 deposition. The interface runs from the left arrow to the right arrow on the micrograph. The interface contains many defects such as voids and cracks which can also be observed in the initial TiN layer. The TiN coating consists of grains
for respectively the (111), (200) and (220) lattice plane reflections. These values could be of austenite but, owing to the different shape of these grains compared with the austenite grains in Fig. 4, and taking into account the result of the GXRD characterization, this phase can be identified as either expanded austenite or ‘/-Fe4N. Although no supperlattice plane reflections are observed in the electron diffraction pattern which should point to a y’-Fe4N structure, unambiguous phase identification is not possible based on the GXRD and XTEM study only. In Fig. 7 the RNRA depth profile obtained on a sample coated according to a type 3 deposition is shown. From this figure it is clear that the atomic nitrogen concentration in the TiN coating is about 44 ±3 at.%. At a depth of 0.2 ~.tmthe atomic nitrogen concentration drops to 12.5 ±3 at.%. Because the coating thickness is estimated to be 170 ±20 nm, the latter nitrogen concentration is measured in the substrate material.
4.
__________
______
(type 2 deposition). The heavily grains cubic structure have d-values of 0.211 faulted nm, 0.183 nm,with 0.129 nm
I
_____
~
intrinsic test parameters (translational
speed of the table and load increase) of the scratch tests
z z 0 I—
~
Fig. 6. XTEM image of the interface of a type 3 sample: the insets a and b are electron diffraction patterns of the grains indicated by and b respectively on the micrograph. The interface runs from the lefthand arrow towards the right-hand arrow.
o~0
0.2
~
0
~
0.4
o
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DEPTH (j.a~i)
Fig. 7. RNRA depth profile for a type 3 sample.
00
~5at%)
0.8
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Interface structure and adhesion
231
are the same for all the samples tested, a direct comparison can be made between the scratches observed for the different types of TiN depositions when the TiN coating
observed with XTEM (Fig. 6) at the interface between the TiN coating and the bulk of the austenitic substrate. These grains are either expanded austenite or ~“-Fe4N.
thickness is the same. From Fig. 1 it is clear that the adhesion of a 1460 nm thick TiN coating is different for the three deposition types. Indeed for a type 3 deposition the coating is removed from the substrate already at a very low load (less than 5 N). In contrast, for types 1 and 2 deposition the coating is well adherent although some differences can be observed. The most important difference is the appearance of semicircular cracks in front of the indenter at lower loads for type 2 deposition than for type 1 deposition. These semicircular cracks are caused by bending of the TiN coating in front of the moving stylus [12]. The results obtained for the three types of deposition are in good agreement with those reported earlier [6]. From Fig. 2 it follows that a similar general trend is observed for the 170 nm thick coating, except that owing to the lower coating thickness no semicircular cracks are observed in the scratches. The XTEM study shows clear differences in interface structure for the different types of deposition. In a type 1 deposition an intermediate Ti layer can be observed which is not pure Ti. The presumable presence of TiO grains, which is probably due to deoxidation of the substrate surface, can thus promote coating adhesion [4]. A type 2 deposition shows a defect-free interface between the TiN coating and AISI 304. In contrast to the type 2 deposition, for type 3 deposition a heavily faulted interface and defects in the initial TiN coating can be observed which result from the initial plasma nitriding of the substrate material. The poor coating adhesion observed for type 3 deposition can be correlated with the formation of this highly defected interface. The unidentified lattice plane reflections observed with GXRD (Fig. 5) are ascribed to the heavily faulted grains
Although the crystallographic difference between expanded austenite and y’-Fe4N [13, 14] is well established, it remains difficult to distinguish between them. Indeed in both phases the nitrogen atoms are in interstitial octahedral positions but the difference between the phases is the positioning of the nitrogen atoms in the crystallographic lattice. These atoms are ordered in y’-Fe4N and are randomly distributed in expanded austenite. This distribution leads to a primitive cubic lattice for the former phase and to an f.c.c. lattice for the latter phase. Although both phases have a different crystallographic structure it is not possible to distinguish between them using an electron diffraction pattern if no superlattice plane electron diffraction spots are clearly observable. It is however possible, using TEM characterization with the convergent beam electron diffraction (CBED) technique, to distinguish between expanded austenite and y’-Fe4N if the phases are not disturbed. Indeed expanded austenite belongs to point group Fm3m and y’-Fe4N belongs to point group P43m. These point groups correspond to different CBED patterns for a specific zone axis. As a consequence, CBED patterns can be used for phase identification. However, the heavily faulted interface grains in this study disturb the symmetry of the CBED patterns which complicates the distinction between the phases. If an equilibrium Fe—N phase is formed at the interface then determination of the atomic nitrogen concentration can be used to distinguish between the phases. Indeed if y’-Fe4N is formed, an atomic nitrogen concentration of 20.00 ±0.15 at.% must be measured at the interface. From Fig. 7 it is clear that the atomic nitrogen concentration at the interface is only 12.5 ±3 at.%. Therefore
20ob~’
A20, 20caic, dhk, and a
TABLE 2.
0 for the lattice plane reflections in Fig. 5 (0~= 1.5°±0.05~) ~ (deg)
A20 (deg)
2Ocaic (deg)
dhkf (nm)
a0 (nm)
67.08 ±0.03 79.20 ±0.03 128.75 ±0.03
0.12 0.12 0.12
66.96 ±0.03 79.08 ±0.03 128.63 ±0.03
0.2077 ±0.0001 0.17994 ±0.00006 0.12711 ±0.00002
0.3598 ±0.0002 0.3599 ±0.0001 0.35952 ±0.00006
Expanded austenite (III) 66.04 ±0.05 (200) 78.08 ±0.04 (220) 125.64 ±0.10
0.12 0.12 0.12
65.92 ±0.05 77.96 ±0.04 125.52 ±0.10
0.2105 ±0.0001 0.1821 ±0.0001 0.12884 ±0.00006
0.3647 ±0.0003 0.3642 ±0.0002 0.3643 ±0.0001
TiN (111) (200) (220) (311) (222)
0.08 0.08 0.08 0.08 0.09
55.35 ±0.03 65.21 ±0.10 99.77 ±0.03 128.57±0.10 142.18 ±0.05
0.2466 ±0.0001 0.2126 ±0.0003 0.14978 ±0.00003 0.1271 ±0.0002 0.1211 ±0.0002
0.4271 ±0.0002 0.4252 ±0.0006 0.42364 ±0.00008 0.4215±0.0008 0.4195 ±0.0007
AISI 304 (111) (200) (220)
55.43 ±0.03 65.29 ±0.10 99.85 ±0.03 128.65±0.10 142.27 ±0.05
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Interface structure and adhesion
the unidentified phase in Fig. 5 is attributed to expanded austenite. In Table 2 the lattice plane distances are given after deconvolution of the observed lattice plane reflections 20obs of Fig. 5, for AISI 304, expanded austenite and TiN. The lattice plane distances dhkj are calculated using the corrected 28 values 20caic~ The latter were determined as explained in ref. 9. In Table 2 the correction factor (A20 = 20caic) is also given. From Table 2 it is clear that different lattice parameters a 0 are obtained for the different lattice planes. The differences in a0 can probably be attributed to strains present in the TiN coating and the austenitic substrate. —
Kernwetenschappen, for financial support under Contract 4.0004.91. Financial support of the Flemish Region, the Wallone Region, the Brussels Capital Region and the Belgian Science Supporting Institute, Instituut ter Bevordering van het Wetenschappelijk Onderzoek in Nijverheid en Landbouw, under Contract CI 1/47674/080 is acknowledged. This text presents research results of the Belgian programme on Interuniversity poles of attraction initiated by the Belgian State, Prime Minister’s Office—Science Policy Programming. Scientific responsibility is assumed by the authors.
References 5. Conclusions 1 D. S. Rickerby and R. B. Newbery, Proc. 6th
By means of glancing angle X-ray diffraction and cross-sectional transmission electron microscopy it was possible to reveal different interface structures for different types of TiN deposition. The poor coating adhesion of TiN on steel substrates generally observed when no intermediate Ti layer is deposited could be attributed to the formation of a highly faulted interface due to the initial plasma nitriding of the substrates. The phase formed at the interface between AISI 304 and TiN due to plasma nitriding of the substrate could be identified as expanded austenite on the basis of measurements of the nitrogen concentration using resonant nuclear reaction analysis. In contrast to what is generally expected, a well adherent TiN coating can be deposited onto austenitic stainless steel without an intermediate Ti layer when the substrate material is not plasma nitrided and a defect-free interface is formed.
Acknowledgments Thanks are due to the Belgian Science Supporting Institute, Interuniversitair Instituut voor
2 3 4 5 6
mt.
Conf on Ion and
Plasma Assisted Techniques, Brighton, 1987, CEP Consultants, Edinburgh, 1987, p. 224. C. C. Cheng, A. Erdemir and G. R. Fenske, Surf. Coat. Technol., 39—40 (1989) 365. U. Helmersson, B. 0. Johansson, J. E. Sundgren, H. T. G. Hentzell and P. Billgren, J. Vac. Sci. Technol. A, 3(2) (1985) 308. M. Van Stappen, B. Malliet, L. De Schepper, L. M. Stals, J. P. Celis and J. R. Roos, Surf. Eng., 4 (1989) 305. C. Quaeyhaegens, L. M. Stals, M. Van Stappen and L. De Schepper, Thin Solid Films, 197 (1991) 37. C. Quaeyhaegens, L. M. Stals, L. De Schepper, M. Van Stappen
and B. Malliet, Surf Coat. Technol., 45 (1991) 193. 7 C. Quaeyhaegens, J. D’Haen, M. Van Stappen, L. De Schepper and L. M. Stals, Proc. 8th Int. Conf. on Ion and Plasma Assisted Techniques, Brussels, 1991, CEP Consultants, Edinburgh, 1991, ~. 296. 8 J. D’Haen, L. De Schepper, M. Van Stappen and L. M. Stals, Micron Microsc. Acta, 21(4) (1990) 279—280. 9 G. Lim, W. Parrish, C. Ortiz, M. Bellotto and M. Hart, J. Mater. Res., 2(4) (1987) 471. 10 U. Helmersson and J. E. Sundgren, J. Electron Microsc. Tech., 4 (1986) 361. 11 C. Quaeyhaegens, M. Van Stappen, L. M. Stals, F. Bodart, G. Terwagne and R. Vlaeminck, Surf Coat. Technol, 54—55 (1992) 279. 12 P. j. Burnett and D. S. Rickerby, Thin Solid Films, 154 (1987) 403. 13 K. H. Jack, Proc. R. Soc. London, Ser, A, 195 (1948) 34. 14 K. H. Jack, Proc. R. Soc. London, Ser, A, 208 (1951) 200.