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Production and characterization of boron nitride films obtained by R.F. magnetron sputtering and reactive ion-beam-assisted deposition
Surface and Coatings Technology,
36 (1988)
199
199 - 206
PRODUCTION AND CHARACTERIZATION OF BORON NITRIDE FILMS OBTAINED BY R.F. MAGNETRON SPUTTERING AND REACTIVE ION-BEAM-ASSISTED DEPOSITION* M. ELENA and L. GUZMAN Zstituto per la Ricerca Scientifica
e Tecnologica,
38050 Povo, Trento
(Italy)
S. GIALANELLA Dipartimento
di Zngegneria, Universita’di
Trento,
38100
Trento
(Italy)
(Received March 1,198s)
BN films were produced on hard metal (WC-Co) substrates using (i) r.f. magnetron sputtering from a BN target and (ii) ion-beam-assisted deposition of boron. The films were characterized using scanning electron microscopy, Auger electron spectroscopy, microhardness and friction measurements and scratch tests in order to study the influence of the deposition process and parameters on the mechanical properties of the films. We determined the best parameters for obtaining hard BN films by r.f. sputtering in our experimental system; we also found that the adhesion can be considerably enhanced by ion-beam-assisted deposition.
1. Introduction Hard and protective coatings are normally applied either by chemical vapour deposition (CVD) or by such physical methods as ion plating or other ion-assisted processes. Although TiN and TiC are still the most widely used materials for hard coatings, in many laboratories efforts are being made to develop new coating materials, in particular new nitrides and carbides. In this context, both r.f. sputtering and ion-beam-assisted deposition techniques can have an important role. The main advantage of r.f. sputtering is that it can be used to produce films of electrically insulating materials. Moreover sputtering techniques have the capability of producing coatings that uniformly cover substrates of complex geometry. R.f. sputtering, however, may present some problems, e.g. adhesion, which can nevertheless be reduced or eliminated with the help of other procedures. *Paper presented at the 15th International San Diego, CA, U.S.A., April 11 - 15, 1988. 0257-8972/88/$3.50
Conference
on Metallurgical Coatings,
@ Elsevier Sequoia/Printed
in The Netherlands
200
The idea of enhancing the properties of a physically deposited film by ion implantation has already been discussed, e.g. by Weissmantel [l, 21 and Dearnaley et al. [ 31. One of these processes is simple ion mixing, where a coating is first deposited and subsequently ion implanted. However, this has certain limitations, owing to the relatively small penetration depth of the accelerated particles produced in the ion implanters generally used for metallurgical purposes. In order to achieve thicker modified layers and higher implanted concentrations, new processes involving both film deposition and ion implantation are being developed. Among these a multiple sequence of deposition-implantation steps is very promising, especially in view of the fact that the depth of the treated region becomes nearly independent of the projected range of the implanted ions. Moreover the ion beam plays an important role in increasing the adhesion and uniformity of the deposited film, and eventually in converting it into a chemically different material, e.g. a nitride. In this paper we report, in particular, on the mechanical characterization (microhardness, scratch and friction measurements) of BN films deposited onto WC-Co substrates. As is well known, BN is an extremely interesting material, from both practical and fundamental points of view, owing to its low density, high thermal conductivity, chemical inertness, hardness etc. BN may be obtained in at least two forms: a cubic zinc blende and a hexagonal structure. Because of the considerable technological importance of BN in many areas, much work has been done to synthesize this material in thin film form. However, it is difficult to obtain at will a specific phase. 2. Preparation of BN coatings All our WC-Co samples, characterized by a Knoop microhardness of 2600 (measured at a load of 25 gf), were mechanically polished to a mirrorlike finish using 3 pm diamond powder in the final step. BN films were deposited using two techniques: (i) r.f. magnetron sputtering from a BN cathode; (ii) a sequence of steps consisting of boron deposition followed by N+ implantation. Some samples were prepared in an Alcatel machine by sputtering in an argon atmosphere using a round BN target 10 cm in diameter. The samples were placed on a heating stage capable of being heated to 400 “C. Variations in the argon pressure (from 0.3 to 0.8 Pa) as well as in the substrate temperature (from 100 to 400 “C) were made in order to study the variation in the mechanical characteristics of BN films. The base pressure before the start of the sputtering process was 5 X 10m5Pa. The deposition conditions are summarized in Table 1. The remaining samples were coated using reactive ion-beam-assisted deposition (RIBAD). BN was synthesized by using two different and independently controlled beams, one from a low energy (30 keV) horizontal ion implanter, the other from an electron beam evaporator installed in the implantation chamber. Specific details about the experimental set-up are given elsewhere [ 41.
201 TABLE 1 Deposition conditions for BN films on WC-Co Technique Target material Base pressure (Pa) Sputtering atmosphere Ar pressure (Pa) Substrate material Substrate potential Substrate temperature (“C) R.f. film growth rate (nm s-l) B deposition rate (RIBAD) (nm s-l) Coating thickness (pm)
R.f. magnetron sputtering BN 5 x 10-s Ar 0. 3 - 0.8 WC-CO Grounded 100 - 400 0.1 ==l
RIBAD B 3 x 10-e WC-CO Grounded Room temperature 0.1 mo.3
The RIBAD process started with an N+ implantation at a dose of (1 - 4) X lOi ions cme2, intended to sputter clean the substrate surface and to create favourable sites for deposition. An additional advantage of this procedure was a strengthening of the WC-Co substrates [3]. The compound was then produced by first depositing a boron layer to a thickness of 27 nm. This value matched the projected range of the ions to be implanted, thus allowing good mixing with the underlying layer, resulting in enhanced adhesion. In order to achieve a stoichiometric composition we then implanted N+ at a dose of 3 X lOi’ ions cmm2, equal to the expected number of atoms present in the deposited film. This dose may have to be corrected for the sputter losses, which have been evaluated to be less than 10%. To obtain the final coating thickness of about 300 nm, ten of the steps described were made. It was not possible to prepare thicker films owing to the limited volume of the electron beam gun crucibles used in this experiment. All the RIBAD processes were performed under controlled conditions in a high vacuum environment (base pressure, 3 X 10M6Pa).
3. Surface microanalyses and mechanical tests 3.1. Microstructure and composition The microstructure of the samples was examined by scanning electron microscopy (SEM). The surface is rather smooth, and the film cross-section shows a compact structure (Fig. 1). Depth-composition profiles were obtained by Auger electron spectroscopy (AES) combined with 2 keV Ar+ sputter etching. The AES depth profiles were determined using the sensitivity factors method described in ref. 5. Since interference occurs between one of the tungsten lines and that of boron at 179 eV, we used other tungsten lines to determine separately the boron and tungsten contributions at the interface.
’
(b)
Fig. 1. Scanning electron micrographs showing the fracture surface of (a) an r.f. magnetron-sputtered BN coating and (b) an RIBAD coating on WC-Co substrates.
I I
I
I I I I
75
P 2
so
-v x 8 = a
25
0 (a)
20
60
100
DEPTH ( nm)
140
160
@I
Fig. 2. (a) AElS depth profiles for a WC-Co sample on which a thin RIBAD coating was deposited by a sequence of process steps, each consisting of boron evaporation and N+ implantation. For clarity, the curves for tungsten and cobalt have not been included in the fire. (b) Auger line shapes for boron in the various film regions (see text).
The film composition was nearly stoichiometric through the entire film thickness for the sputtered samples, whereas three clearly defined regions are present in the case of ion-beam-assisted coatings. Figure 2(a) shows the sputter profiles for a sample obtained by ten sequential steps each consisting of boron evaporation and N+ implantation. Throughout the profile the Auger line shapes of boron and nitrogen were also recorded. The depth
203
profiles indicate the formation of a BN layer (region b) just below a surface layer of “untransformed” boron (region a) and just above a broad ionmixed interface (region c), which should ensure excellent adhesion. The film is nitrogen deficient; the AES line shape shown in Fig. 2(b) is representative of elemental boron in the near-surface region (a), while under the surface it is typically that of BN even if the N:B ratio attains only the value 2:3 over an extended depth. The coating shows a yellowish hue, similarly to other cubic nitrides which are golden in color. At present, however, we have no conclusive information about its crystallographic structure. Some preliminary data are indicative of a cubic structure; indeed it is otherwise difficult to account for the microhardness values obtained. 3.2. Scmtch and friction testing In the scratch tests a loaded stylus was drawn repeatedly across the surface of the coated samples. The stylus radius was 200 pm and the following loads were applied: 2, 3, 4, 6, 8, 10 and 20 N. Figure 3 is an optical micrograph showing morphological features of the scratches, which are different for the two deposition techniques employed. The depth of the scratches was measured by means of a profilometer; the data taken at very low loads (2 N) were found to simulate correctly the microhardness data (see Figs. 6 and 7) and to have the advantage of not requiring the optical
Fig. 3. Optical micrographs showing the scratches for (a) r.f.-sputtered and (b) RIBAD samples (scratch load, 4 N; bar, 50 Pm long). The different coating failure mechanisms should be noted.
204
measurement of small indents. Moreover the critical load for film detachment was deduced from optical microscopy as well as from an acoustic emission signal recorded during the scratch test. The most obvious difference between the sputtered coatings and those obtained by RIBAD is that, while a critical load L, can definitely be measured in the former case (where the film suddenly detaches from the substrate), in the latter no such decohesion is present even if the substrate is heavily deformed at sufficiently high loads. Figures 4 and 5 show both L, and the friction coefficient /.Las a function of the substrate temperature and the argon pressure respectively; they are a useful indication of the optimum deposition parameters: 300 “C and 0.7 Pa. A minimum friction coefficient /J = 0.05 is found, to be compared with the values of 0.15 - 0.30 obtained for other samples. 3.3. Hardness testing The BN-coated samples obtained by r.f. sputtering were hardness tested using a Knoop indenter with a load of 25 gf to give an impression depth between 250 and 400 nm, i.e. well within the BN film. Figure 6 shows the hardness as a function of substrate temperature, for an argon pressure of 0.5 Pa in the chamber. Six indenter readings were taken for each point on I P, 0.3 -
_RIBAD
150w
T, 3009:
.
P,lSOW 0.2
J?IBAD
Ar
pnrsur. 5 0.5 R
m 5 0.2 e x 0 f, F
O.l-
0 z IA
0.1
I
0.3
I
0.5
PRESSURE
I
0.9 (Pa)
Fig. 4. Friction coefficient (w, measured at a normal load of 2N) and critical load (A) for film detachment from the substrate as a function of substrate temperature, for samples obtained by r.f. sputtering at a power of 150 W and an argon pressure of 5 x 10-i Pa. The values for an RIBAD film are also shown for comparison by arrows along ordinate axes. Fig. 5. Friction coefficient (m, measured at a normal load of 2N) and critical load (A) for film detachment from the substrate as a function of argon pressure in the chamber for samples obtained by r.f. sputtering at a power of 150 W. The values for an RIBAD film are also shown for comparison.
205
P=lSO
w
Ar pressure=m
100
200
300
TEMPERATURE
Pa
400
(“C>
Fig. 6. Knoop microhardness (m) as a function of substrate temperature for r.f. magnetronsputtered BN coatings. The indentation depths for scratches at a load of 2N are also shown (A).
P.
150
w
T=
300
oc
I’4 :-I
0.3
0.6
PRESSURE
0.9
( Pa 1
Fig. 7. Knoop microhardness (B) as a function of argon pressure in the vacuum chamber for r.f. magnetron-sputtered BN coatings. The indentation depths for scratches at a load of 2N are also shown (A).
the graphs, which represent average values together with standard deviations. No strong dependence of hardness on substrate temperature was found. Figure 7 shows hardness as a function of argon pressure in the vacuum chamber for a substrate temperature of 300 “C. In this case the optimum pressure was found to be 0.7 Pa, giving the highest hardness values.
206
4. Discussion
Both r.f. magnetron sputtering and RIBAD are able to produce hard BN coatings. In the case of r.f.-sputtered samples, the improvement in hardness with respect to the substrate is significant as the hardnesses obtained are higher than 3500 HK. This very good value is confirmed by the very small scratch depths measured by means of profilometry. Moreover the friction coefficients p measured during the scratch tests were’ found to be of the order of 0.05, i.e. six times lower than the corresponding p values for the substrates against the same diamond stylus. A possible explanation for this improvement may be the observed enhanced film roughness, which may reduce the contact area between the stylus and the coating. However, these p values are not especially low as would be expected on the basis of the chemical inertness and hardness of BN. This complicated matter deserves further study in order to ascertain the role of the various mechanisms involved [ 61. The critical load for film detachment shows a maximum value in accord with the best hardness and friction coefficient values. In fact, one of the most critical aspects of the coatings is their adhesion, which is clearly related to the deposition technique. 5. Conclusion The best parameters for obtaining hard BN films by r.f. sputtering in the system described were determined. The adhesion can be considerably enhanced by RIBAD, owing to the absence of a sharp interface, as indicated by the AES depth analysis. In this case BN coatings are observed to be adherent even after substrate failure. Acknowledgment We gratefully acknowledge the help of Dr. Maurizio Dapor of I.R.S.T. for obtaining the Auger data and for the interesting discussions.
References 1 C. Weissmantel, Vide, Couches Minces, 41 (1986) 45. 2 C. Weissmantel, Vide, Couches Minces, Suppl., 237 (1987) 44. 3 G. Dearnaley, P. D. Goode, F. J. Minter, A. T. Peacock and C. N. Waddel, J. Vat. Sci. Technol., A, 3 (1985) 2684. 4 B. Margesin, F. Giacomozzi, L. Guzman, G. Lazzari and V. Zanini, Nucl. Instrum. Methods B, 21 (1987) 566. 5 L. E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach and R. E. Weber, Hundbook of Auger Electron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1978. 6 E. Rabinowicz, Friction and Wear of Materials, Wiley, New York, 1965, p. 82.
36 (1988)
199
199 - 206
PRODUCTION AND CHARACTERIZATION OF BORON NITRIDE FILMS OBTAINED BY R.F. MAGNETRON SPUTTERING AND REACTIVE ION-BEAM-ASSISTED DEPOSITION* M. ELENA and L. GUZMAN Zstituto per la Ricerca Scientifica
e Tecnologica,
38050 Povo, Trento
(Italy)
S. GIALANELLA Dipartimento
di Zngegneria, Universita’di
Trento,
38100
Trento
(Italy)
(Received March 1,198s)
BN films were produced on hard metal (WC-Co) substrates using (i) r.f. magnetron sputtering from a BN target and (ii) ion-beam-assisted deposition of boron. The films were characterized using scanning electron microscopy, Auger electron spectroscopy, microhardness and friction measurements and scratch tests in order to study the influence of the deposition process and parameters on the mechanical properties of the films. We determined the best parameters for obtaining hard BN films by r.f. sputtering in our experimental system; we also found that the adhesion can be considerably enhanced by ion-beam-assisted deposition.
1. Introduction Hard and protective coatings are normally applied either by chemical vapour deposition (CVD) or by such physical methods as ion plating or other ion-assisted processes. Although TiN and TiC are still the most widely used materials for hard coatings, in many laboratories efforts are being made to develop new coating materials, in particular new nitrides and carbides. In this context, both r.f. sputtering and ion-beam-assisted deposition techniques can have an important role. The main advantage of r.f. sputtering is that it can be used to produce films of electrically insulating materials. Moreover sputtering techniques have the capability of producing coatings that uniformly cover substrates of complex geometry. R.f. sputtering, however, may present some problems, e.g. adhesion, which can nevertheless be reduced or eliminated with the help of other procedures. *Paper presented at the 15th International San Diego, CA, U.S.A., April 11 - 15, 1988. 0257-8972/88/$3.50
Conference
on Metallurgical Coatings,
@ Elsevier Sequoia/Printed
in The Netherlands
200
The idea of enhancing the properties of a physically deposited film by ion implantation has already been discussed, e.g. by Weissmantel [l, 21 and Dearnaley et al. [ 31. One of these processes is simple ion mixing, where a coating is first deposited and subsequently ion implanted. However, this has certain limitations, owing to the relatively small penetration depth of the accelerated particles produced in the ion implanters generally used for metallurgical purposes. In order to achieve thicker modified layers and higher implanted concentrations, new processes involving both film deposition and ion implantation are being developed. Among these a multiple sequence of deposition-implantation steps is very promising, especially in view of the fact that the depth of the treated region becomes nearly independent of the projected range of the implanted ions. Moreover the ion beam plays an important role in increasing the adhesion and uniformity of the deposited film, and eventually in converting it into a chemically different material, e.g. a nitride. In this paper we report, in particular, on the mechanical characterization (microhardness, scratch and friction measurements) of BN films deposited onto WC-Co substrates. As is well known, BN is an extremely interesting material, from both practical and fundamental points of view, owing to its low density, high thermal conductivity, chemical inertness, hardness etc. BN may be obtained in at least two forms: a cubic zinc blende and a hexagonal structure. Because of the considerable technological importance of BN in many areas, much work has been done to synthesize this material in thin film form. However, it is difficult to obtain at will a specific phase. 2. Preparation of BN coatings All our WC-Co samples, characterized by a Knoop microhardness of 2600 (measured at a load of 25 gf), were mechanically polished to a mirrorlike finish using 3 pm diamond powder in the final step. BN films were deposited using two techniques: (i) r.f. magnetron sputtering from a BN cathode; (ii) a sequence of steps consisting of boron deposition followed by N+ implantation. Some samples were prepared in an Alcatel machine by sputtering in an argon atmosphere using a round BN target 10 cm in diameter. The samples were placed on a heating stage capable of being heated to 400 “C. Variations in the argon pressure (from 0.3 to 0.8 Pa) as well as in the substrate temperature (from 100 to 400 “C) were made in order to study the variation in the mechanical characteristics of BN films. The base pressure before the start of the sputtering process was 5 X 10m5Pa. The deposition conditions are summarized in Table 1. The remaining samples were coated using reactive ion-beam-assisted deposition (RIBAD). BN was synthesized by using two different and independently controlled beams, one from a low energy (30 keV) horizontal ion implanter, the other from an electron beam evaporator installed in the implantation chamber. Specific details about the experimental set-up are given elsewhere [ 41.
201 TABLE 1 Deposition conditions for BN films on WC-Co Technique Target material Base pressure (Pa) Sputtering atmosphere Ar pressure (Pa) Substrate material Substrate potential Substrate temperature (“C) R.f. film growth rate (nm s-l) B deposition rate (RIBAD) (nm s-l) Coating thickness (pm)
R.f. magnetron sputtering BN 5 x 10-s Ar 0. 3 - 0.8 WC-CO Grounded 100 - 400 0.1 ==l
RIBAD B 3 x 10-e WC-CO Grounded Room temperature 0.1 mo.3
The RIBAD process started with an N+ implantation at a dose of (1 - 4) X lOi ions cme2, intended to sputter clean the substrate surface and to create favourable sites for deposition. An additional advantage of this procedure was a strengthening of the WC-Co substrates [3]. The compound was then produced by first depositing a boron layer to a thickness of 27 nm. This value matched the projected range of the ions to be implanted, thus allowing good mixing with the underlying layer, resulting in enhanced adhesion. In order to achieve a stoichiometric composition we then implanted N+ at a dose of 3 X lOi’ ions cmm2, equal to the expected number of atoms present in the deposited film. This dose may have to be corrected for the sputter losses, which have been evaluated to be less than 10%. To obtain the final coating thickness of about 300 nm, ten of the steps described were made. It was not possible to prepare thicker films owing to the limited volume of the electron beam gun crucibles used in this experiment. All the RIBAD processes were performed under controlled conditions in a high vacuum environment (base pressure, 3 X 10M6Pa).
3. Surface microanalyses and mechanical tests 3.1. Microstructure and composition The microstructure of the samples was examined by scanning electron microscopy (SEM). The surface is rather smooth, and the film cross-section shows a compact structure (Fig. 1). Depth-composition profiles were obtained by Auger electron spectroscopy (AES) combined with 2 keV Ar+ sputter etching. The AES depth profiles were determined using the sensitivity factors method described in ref. 5. Since interference occurs between one of the tungsten lines and that of boron at 179 eV, we used other tungsten lines to determine separately the boron and tungsten contributions at the interface.
’
(b)
Fig. 1. Scanning electron micrographs showing the fracture surface of (a) an r.f. magnetron-sputtered BN coating and (b) an RIBAD coating on WC-Co substrates.
I I
I
I I I I
75
P 2
so
-v x 8 = a
25
0 (a)
20
60
100
DEPTH ( nm)
140
160
@I
Fig. 2. (a) AElS depth profiles for a WC-Co sample on which a thin RIBAD coating was deposited by a sequence of process steps, each consisting of boron evaporation and N+ implantation. For clarity, the curves for tungsten and cobalt have not been included in the fire. (b) Auger line shapes for boron in the various film regions (see text).
The film composition was nearly stoichiometric through the entire film thickness for the sputtered samples, whereas three clearly defined regions are present in the case of ion-beam-assisted coatings. Figure 2(a) shows the sputter profiles for a sample obtained by ten sequential steps each consisting of boron evaporation and N+ implantation. Throughout the profile the Auger line shapes of boron and nitrogen were also recorded. The depth
203
profiles indicate the formation of a BN layer (region b) just below a surface layer of “untransformed” boron (region a) and just above a broad ionmixed interface (region c), which should ensure excellent adhesion. The film is nitrogen deficient; the AES line shape shown in Fig. 2(b) is representative of elemental boron in the near-surface region (a), while under the surface it is typically that of BN even if the N:B ratio attains only the value 2:3 over an extended depth. The coating shows a yellowish hue, similarly to other cubic nitrides which are golden in color. At present, however, we have no conclusive information about its crystallographic structure. Some preliminary data are indicative of a cubic structure; indeed it is otherwise difficult to account for the microhardness values obtained. 3.2. Scmtch and friction testing In the scratch tests a loaded stylus was drawn repeatedly across the surface of the coated samples. The stylus radius was 200 pm and the following loads were applied: 2, 3, 4, 6, 8, 10 and 20 N. Figure 3 is an optical micrograph showing morphological features of the scratches, which are different for the two deposition techniques employed. The depth of the scratches was measured by means of a profilometer; the data taken at very low loads (2 N) were found to simulate correctly the microhardness data (see Figs. 6 and 7) and to have the advantage of not requiring the optical
Fig. 3. Optical micrographs showing the scratches for (a) r.f.-sputtered and (b) RIBAD samples (scratch load, 4 N; bar, 50 Pm long). The different coating failure mechanisms should be noted.
204
measurement of small indents. Moreover the critical load for film detachment was deduced from optical microscopy as well as from an acoustic emission signal recorded during the scratch test. The most obvious difference between the sputtered coatings and those obtained by RIBAD is that, while a critical load L, can definitely be measured in the former case (where the film suddenly detaches from the substrate), in the latter no such decohesion is present even if the substrate is heavily deformed at sufficiently high loads. Figures 4 and 5 show both L, and the friction coefficient /.Las a function of the substrate temperature and the argon pressure respectively; they are a useful indication of the optimum deposition parameters: 300 “C and 0.7 Pa. A minimum friction coefficient /J = 0.05 is found, to be compared with the values of 0.15 - 0.30 obtained for other samples. 3.3. Hardness testing The BN-coated samples obtained by r.f. sputtering were hardness tested using a Knoop indenter with a load of 25 gf to give an impression depth between 250 and 400 nm, i.e. well within the BN film. Figure 6 shows the hardness as a function of substrate temperature, for an argon pressure of 0.5 Pa in the chamber. Six indenter readings were taken for each point on I P, 0.3 -
_RIBAD
150w
T, 3009:
.
P,lSOW 0.2
J?IBAD
Ar
pnrsur. 5 0.5 R
m 5 0.2 e x 0 f, F
O.l-
0 z IA
0.1
I
0.3
I
0.5
PRESSURE
I
0.9 (Pa)
Fig. 4. Friction coefficient (w, measured at a normal load of 2N) and critical load (A) for film detachment from the substrate as a function of substrate temperature, for samples obtained by r.f. sputtering at a power of 150 W and an argon pressure of 5 x 10-i Pa. The values for an RIBAD film are also shown for comparison by arrows along ordinate axes. Fig. 5. Friction coefficient (m, measured at a normal load of 2N) and critical load (A) for film detachment from the substrate as a function of argon pressure in the chamber for samples obtained by r.f. sputtering at a power of 150 W. The values for an RIBAD film are also shown for comparison.
205
P=lSO
w
Ar pressure=m
100
200
300
TEMPERATURE
Pa
400
(“C>
Fig. 6. Knoop microhardness (m) as a function of substrate temperature for r.f. magnetronsputtered BN coatings. The indentation depths for scratches at a load of 2N are also shown (A).
P.
150
w
T=
300
oc
I’4 :-I
0.3
0.6
PRESSURE
0.9
( Pa 1
Fig. 7. Knoop microhardness (B) as a function of argon pressure in the vacuum chamber for r.f. magnetron-sputtered BN coatings. The indentation depths for scratches at a load of 2N are also shown (A).
the graphs, which represent average values together with standard deviations. No strong dependence of hardness on substrate temperature was found. Figure 7 shows hardness as a function of argon pressure in the vacuum chamber for a substrate temperature of 300 “C. In this case the optimum pressure was found to be 0.7 Pa, giving the highest hardness values.
206
4. Discussion
Both r.f. magnetron sputtering and RIBAD are able to produce hard BN coatings. In the case of r.f.-sputtered samples, the improvement in hardness with respect to the substrate is significant as the hardnesses obtained are higher than 3500 HK. This very good value is confirmed by the very small scratch depths measured by means of profilometry. Moreover the friction coefficients p measured during the scratch tests were’ found to be of the order of 0.05, i.e. six times lower than the corresponding p values for the substrates against the same diamond stylus. A possible explanation for this improvement may be the observed enhanced film roughness, which may reduce the contact area between the stylus and the coating. However, these p values are not especially low as would be expected on the basis of the chemical inertness and hardness of BN. This complicated matter deserves further study in order to ascertain the role of the various mechanisms involved [ 61. The critical load for film detachment shows a maximum value in accord with the best hardness and friction coefficient values. In fact, one of the most critical aspects of the coatings is their adhesion, which is clearly related to the deposition technique. 5. Conclusion The best parameters for obtaining hard BN films by r.f. sputtering in the system described were determined. The adhesion can be considerably enhanced by RIBAD, owing to the absence of a sharp interface, as indicated by the AES depth analysis. In this case BN coatings are observed to be adherent even after substrate failure. Acknowledgment We gratefully acknowledge the help of Dr. Maurizio Dapor of I.R.S.T. for obtaining the Auger data and for the interesting discussions.
References 1 C. Weissmantel, Vide, Couches Minces, 41 (1986) 45. 2 C. Weissmantel, Vide, Couches Minces, Suppl., 237 (1987) 44. 3 G. Dearnaley, P. D. Goode, F. J. Minter, A. T. Peacock and C. N. Waddel, J. Vat. Sci. Technol., A, 3 (1985) 2684. 4 B. Margesin, F. Giacomozzi, L. Guzman, G. Lazzari and V. Zanini, Nucl. Instrum. Methods B, 21 (1987) 566. 5 L. E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach and R. E. Weber, Hundbook of Auger Electron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1978. 6 E. Rabinowicz, Friction and Wear of Materials, Wiley, New York, 1965, p. 82.