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Effect of sandblasting on adhesion strength of diamond coatings
~7-70
ELSEVIER
Thin Solid Films 307 (1997) 21-28
J
7"7
ll/H$
Effect of sandblasting on adhesion strength of diamond coatings Bi Zhang *, Lanying Zhou Department of Mechanical Engineering, Precisioff Mam(acturing bzsfitute, Unicersit), of Connecticut, S~orrs, 06269-5119, CT, USA
Received 9 December I996: accepted 13 May i997
Abstract Premature failure of a diamond-coated tool often results from a poor adhesion of the coating and shortens the lifetime of the tool. This study presents the results of increasing the adhesion strength of diamond coatings on cutting tool inserts by pretreating the inserts with sandblasting technique to obtain a desirable surface morphology of the inserts. A geometric model representing the ideal surface morphology is established to enhance the nucleation density and adhesion strength of diamond coatings. Diamond coating experiment is conducted on the substrates of four different sample groups. Indentation and wear tests are performed on diamond-coated inserts to evaluate the effect of sandblasting on the adhesion strength of the coatings. A theoretical analysis is provided on the formation and growth of atom clusters in terms of the contact angle and the thermodynamic barrier of a substrate to predict diamond nucleation. The theoretical prediction has a good agreement with the experimental results obtained in this study and by Dennig and Stevenson [P.A. Dennig, D.A. Stevenson, Proceedings of the International Conference on New Diamond Science and Technology, 403 (1991): P.A. Dennig, D.A. Stevenson, Proceedings of the First International Conference on the Applications of Diamond Films and Related Materials, 383 (1991).]. © 1997 Elsevier Science S.A. Keywords: Adhesion; Coatings; Diamond; Surface morphology
1. Introduction The subject of diamond coating has widely been investigated for the last decade. Dennig and Stevenson studied diamond nucleation on chemically etched silicon substrates, and observed that the majority of nucleation events occurred on protruding surface features [1,2]. Other researchers used grinding and polishing techniques to enhance the nucleation density of diamond [3,4]. In a coating process, control of diamond deposits requires an extensive "knowledge of nucleation enhancement [5]. Not only are the atomic mechanisms for the growth of diamond very important [6,7], but substrate parameters such as defect type and density [8], material surface energy [9], morphology [10], crystal structure [11], and lattice constant [12] are also important, since they can dominate the nucleation behavior of diamond particles on a particular substrate. Many investigators used various techniques to modify the surfaces, and therefore, the nucleation behavior of their substrates. A commonly used method to enhance the nucleation den-
* Corresponding author. Tel.: + 1 860 4861489; fax: + 1 860 4862269; e-mail: [email protected] 0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights rese~'ed. PII S0040-6090(97)00297-6
sity and adhesion strength of diamond coatings is to scratch the surface of a substrate with diamond powders. However, not all means of surface pretreatment are appropriate for diamond coating. Scratching an optical surface with abrasives would damage that surface although it could enhance the nucleation density and growth rate of diamond [13]. The application of the scratching method is, therefore, limited. In a diamond coating process, diamond nucleation usually involves the following sequence: surface adsorption formation of steady-state atom cluster ~ cluster growth nucleus formation. Etching by atomic hydrogen may take place during cluster formation and growth. Concerning cluster growth, one may raise the following questions. At what size do the clusters become diamond nuclei? Can multi-clusters grow simultaneously? Is the process of cluster formation reversible? The aim of this study is to answer the questions through the exploration of the effect of substrate pretreatment on diamond nucleation by varying the surface geometric morphology of a substrate. A geometric model of an ideal surface morphology is proposed, and a mathematical expression representing steady-state atom cluster growth is developed based on the surface free energy of a substrate
B. Zhang, L. Zhou / Thin Solid Fihns 307 (1997) 21-28
22
to predict diamond nucleation. Diamond coating experiment is performed on the substrates pretreated with sandblasting of alumina powders. The diamond coatings deposited in the experiment are then evaluated using indentation and wear tests to verify the theoretical prediction. The substrate pretreatment shows a promising result on the enhancement of adhesion strength of diamond coatings. Fig. 2. G e o m e t r i c model representing an ideal surface m o r p h o l o g y : p y r a m i d s on hemispheres with skewness values b e t w e e n - 1 a n d - 3.
2. Surface morphology model Diamond nucleates on the surface of a substrate in a coating process. When forming a critical nucleus, it is necessary to increase the free energy of the system in which an atom cluster grows. If the nucleus is spherical with a radius r, the free-energy change can be given as 4
Gr = 4"r;rS7 + -~ ~ r 3 Q
(1)
For a nucleus with a small radius, e.g., a cluster of carbon atoms, the first term of the equation predominates. As the size of the atom cluster increases, the free energy required to form it also increases. For any further growth of the cluster, however, the second term tends to be larger than the first one, so that once the cluster reaches a certain critical size, the second term becomes dominant. Further growth of the cluster leads to a decreased free energy requirement, and thus, a steady state growth. The diamond nucleation process can be described by three steps which can be schematically shown in Fig. 1.
C
C
A & D -- platte B & E -- cc~eave C & F -- convex
The first step involves surface adsorption at which atom clusters form on the surface of a substrata at random distribution. The second step consists of cluster formation. From the surface adsorption to cluster formation, the substrata-gas phase interface with a higher free energy (e.g., surface defect, convex or concave surface feature) would favor the cluster formation, which would result in a preferential placement of the atom clusters at points B, C, E, and F as shown in Fig. 1c. The third step is nucleation step. At the nucleation step, the points with the concave surface feature will not favor the further growth of the atom clusters due to a higher free energy requirement, and therefore, the clusters shrink and eventually vanish at points B and E. Conversely, the clusters at points C and F continuously grow to form nuclei, and can grow indefinitely large until they meet each other to form diamond crystals. In general, a diamond coating can be formed uniformly and approximately duplicate the geometric configuration of a substrate surface. Since diamond nucleation is in close relationship with the surface morphology of a substrate, a mode] representing the ideal surface morphology of a substrate is constructed and shown in Fig. 2. In this model, the ideal morphology is a protuberance such as a ridge on a pyramid feature, on which a steady-state atom cluster should be formed favorably. The skewness of the ideal surface morphology can be described by the following equation,
Sk = o'-3 L+°~z3p( z)dz
(a) Subs*ratewith various surface features
(b) Atom adsurbaon on the surface features
(2)
Based on the experimental results obtained in this study, the diamond coatings with the highest nucleation density and adhesion strength were obtained on substrates with
Ptasma torch
• (d) Diamond nucleation on the convex surface feature
Window A-~ubstrate
l (b) Atom cluster formation on the surface features
Fig. 1. Schematics s h o w i n g nucleation process. O n l y the convex surface feature is ideal for d i a m o n d nucleation although atom cluster formation a n d gas adsorption can take place on the surface features.
[--~~--] CH4
H2
To pump
Ar
Fig. 3. Schematic d i a g r a m o f DC p l a s m a apparatus with tungsten a n d cerium as p l a s m a discharge electrodes, methane gas as carbon source. h y d r o g e n as reaction gas a n d argon as protective gas.
B. Zhang, L. Zhou / 77m7 Sohd Fihns 307 (!997) 21-28
Table 1 Experimental conditions for diamond coating Coating time Current Voltage Methane gas flow rate Hydrogen gas flow rate Substrate distance from electrode Substrate temperature Chamber pressure
1.5 h 102 A 30 V 25.0 sccm i580 sccm 6.0 cm 850°C 3000 Pa
pyramids of 3 - 5 /zm in diameter and 0 . 2 - 1 . 0 /xm in depth. By substituting these values into Eq. (2), the skewness value of the ideal surface morphology is then obtained as S k = - 1 - - 3.
3. Experimentation Diamond coating experiments were conducted using a DC Plasma apparatus schematically shown in Fig. 3. The plasma discharge electrode was made of tungsten and cerium. Methane gas was used as a carbon source while hydrogen as reaction gas and argon as protective gas. The experimental conditions are summarized in Table 1. The conditions for the substrate pretreatment were varied, however, the other conditions remained unchanged. The conditions for substrate pretreatment are listed in Table 2. The substrates were first degreased in acetone and ethanol, rinsed in deionized water, and then, sandblasted when necessary. After sandblasting, the substrates were suspended in an ultrasonic bath of ethanol, rinsed in deionized water, finally cleaned in H F acid, and rinsed in deionized water again. The substrate material was tungsten carbide (WC + 6% Co). Samples of four groups were used in the experiment. Sample group S o were ground without sandblasting and diamond coating. Sample group S~ were ground, and then, coated with diamond without sandblasting. Sample group S 2 were ground, blasted with # 1 2 0 alumina sands, and coated with diamond. Sample group S 3 were ground, blasted with # 1 2 0 and # 5 0 0 alumina sands, and coated with diamond. The sandblasting was performed at a pressure of 2 . 5 - 3 k g / c m a for 30 min in water carrier. The adhesion strength of the samples was evaluated using a Rockwell hardness tester, while the wear-resistance of
the samples was tested using a block-on-shaft apparatus. All the diamond coatings were characterized by scanning electron microscopy (SEM) and Raman spectroscopy.
4. Experimental results Fig. 4 shows SEM photographs of the diamond coatings on sample groups S 1 and S 3. The crystal size of the diamond coating deposited on sample group S 1 was larger than that on sample group S 3. The crystal size of sample group S 3 was approximately 3 /xm. while that of sample group S 1 was 6 p.m. Fig. 5 presents Raman spectra of the diamond coatings on sample groups $1 and S 3. No significant difference is identified between the sample groups in terms of Raman peak width at 1334 cm -1 for diamond and 1550 cm -1 for non-diamond. The indentation test results are shown in Fig. 6. At a load of 600 N, coating flaking was observed on sample group S t but not on sample group S 3. In fact, coating flaking on sample group S 3 was not observed until the indentation load was increased to 800 N. The exact failure mechanisms are to be investigated for the sample groups used in this study. However, based on the experimental observations, shear stress induced delamination may be considered as a failure mechanism for the flaking of sample group S 3, while tensile stress induced chipping as a failure mechanism for the flaking of sample group S t . The experimental results showed that the adhesion strength of a diamond coating was increased when a sample was pretreated with sandblasting than that without sandblasting.
Table 2 Tungsten carbide substrates subjected to various pretreatment conditions Sample group
Grinding
# 120 blasting
#500 blasting
Diamond coating
S0 St S2 S3
yes yes yes yes
NflA N/A yes yes
N/A N/A N/A yes
NflA yes yes yes
Fig. 4. SE~VFphotographsof diamond coatings on substrates with and without sandblasting pretreatment,
B, Zfiang, L, Zhou/Thin Solid Films 307 (1997) 21-28
24
30
2.0:
& ×
.5
10I
%
q
V=400 mTmm N=87 N Block: diamondcoated Shaft: tungstencarbide Lubricant: 20# engine oil
25
2o
_
Block,
i!/
$N
o,5 ¢
Shat~
~.
&0 1100
I
I
1
I
I
1200
1300
1400
1500
1600
10
V
1700
Raman shift, cm'l
(a) Sandblasted sample from group g3
n S3
2.0
S,_
St
So
Sample group
x
~.5
0.5
i1 /
Fig. 7, Block-on-shaft wear rate of various samples subjected to respective pretreatment conditions. A decreasing wear rate is observed from the samples pretreated by sandblasting with fine and coarse sands and by grinding.
~'~
0,0
1100
I
I
I
I
I
1200
1300
1400
t 50fl
160i)
1700
Raman shift, cm-1
In addition, the blasting pretreatment with finer sands can provide an enhanced wear-resistance than that with coarser sands.
Co) Ground sample from group S~ Fig. 5. Raman spectra of the diamond coatings, No significant difference identified between the two sample groups.
The wear-resistance test results of different sample groups are shown in Fig. 7. It was found that the wear rate of sample group S 3 was 14 times lower than that of sample group So; 5.5 times lower than that of sample group St: and 2 times lower than that of sample group S 2. The experimental results revealed that the wear-resistance of a diamond coating deposited on a substrate pretreated with sandblasting is much higher than that without sandblasting.
5. D i s c u s s i o n
The effect of the sandblasting pretreatment of substrate on the adhesion strength of diamond coatings has been demonstrated by the experiments. The growth of a polycrystalline diamond film begins, after some incubation time, with surface adsorption on a substrate, which leads to the formation of atom clusters. The free energy necessary to form an atom cluster increases with the increase of the size of the atom cluster. Once a steady-state atom cluster reaches a certain critical size, the corresponding critical free energy is the thermodynamic barrier for nucleation on a substrate. The critical size of the atom cluster and the thermodynamic barrier are expressed by the following equations: -2y =
- -
(3)
16~f(O)~/3 3G~
(4)
where (2 + cos 0 ) ( 1 - cos 0) 2
f(0) =
Fig. 6. Effect of sandblasting on indentation strength of diamond coatings.
4
(5)
An atom cluster having a radius less than the critical size is called a subcritical embryo; and those of the critical size or larger are called critical and supercritical nuclei. The thermodynamic barrier is a function of the surface free energy of the substrate at nucleating sites and contact angle between a nucleus and the substrate. A general trend
B. Zhang, L. Zhou / Thin Solid Fihns 307 (1997) 21-28
of such a nucleating site is to reduce the thermodynamic barrier for nucleation. When a nucleus forms on a substrate, a high energy substrate-gas phase interface is replaced by a lower energy substrate-nucleus interface, thereby, resulting in a smaller overall surface energy contribution. Nucleation would begin at a high energy center, e.g. a defect site, of a substrate. But a different type of high energy centers would result in a different substratenucleus contact angle. The thermodynamic barrier for nucleation decreases with decreasing contact angle and approaches zero as the contact angle approaches zero. The decrease of the thermodynamic barrier facilitates an easy growth of the atom clusters, and thus, the formation of nuclei. A lower thermodynamic bmTier can be obtained from a substrate of the convex surface feature because such a substrate provides a small contact angle for nucleation. Besides, the probability for the atom cluster to convert to a nucleus onto such a surface is much higher than that onto other types of surfaces due to the same reason. Whether an atom cluster can grow to become a nucleus depends on the rates of atom deposition and atom etching. The number of diffusion atoms deposited onto a cluster in a unit time can be written as: Nd
=
( 2 e r r sin O)Noavse -zd/kT
(6)
The etching rate of the atom cluster due to atomic hydrogen is directly proportional to the surface area of the steady-state atom cluster. The number of atoms etched by atomic hydrogen in a unit time is: N H = 2~-r2(1 - cos O)R u
(7)
so the number of atoms contributing to the cluster growth in a unit time is then obtained as:
N=Noav~e-E~/kr(2rrr sin 0) - 2~-r2(1 - cos O)R H
(s) N Ou~e -ed/kr = p(2rrmkr) - t / z
&-a~ 1/6
ff
1/i2
o5
Fig. 8. G r o w t h cmwe for a steady-state a t o m d u s t e r : at t o = a / ( r I ~), the n u m b e r o f atoms in a cluster reaches the m a x i m u m .
Through the first-order approximation, the radius of an atom cluster can be expressed as:
r(t) = r o + rlt
N is then represented by
N = ap(Z~rmkT) -1/22rrr sin 0 - 2~rr2(1 - cos O) R H
N(t) =
(13)
At moment t, the number of atoms within the steadystate atom cluster is given by:
,2(t) = fo~N( t)dt
a = ap(2crmkT)-1/2(2rr sin 0) and /3 = 2~r(1 - cos O)R H
(14)
By substituting Eq. (13) into Eq. (14), the number of atoms contributing to the cluster growth is given as:
n(t) = ---/3r?t3 + 3
0~1"1
2
t2 + C
According to the initial condition of the cluster growth, n(0) = 0 at t = 0, the inte~ation constant is found to be C = 0. Eq. (14) can, therefore, be rewritten as:
~3r--t3 +
C~t'l /,2
(15)
2
Eq. (15) represents the number of atoms contributing to the duster growth in terms of deposition time, which is also plotted in Fig. 8. The maximum number of atoms in a cluster is then found at t o = a / ( / 3 r 1) as:
(10) The atom cluster can grow if N is greater than zero. Otherwise, it may shrink back and vanish due to the etching effect by atomic hydrogen. Let
(12)
with an assumption that the radius is zero when at time zero under the initial conditions, Eq. (I 1) is changed to
3 (9)
1.5
t, [d(~,5):
n(t)
which is also evidenced by Elton [12]. Since
25
[ ap(2 ¢rmkT)- 1/2( 2 73" sin 0 )]3 '/max =
6rl[2~r(1 - cos 0 ) R . ] :
(t6)
Since the thermodynamic barrier is a measure of the free energy required to form a critical nucleus, its value varies with many factors, such as the contact angle between an atom cluster and substrate surface. The number of atoms of a critical nucleus is [14] n* = no e - c • / k r
(17)
then the following equation can be obtained
N( t) = a r ( t ) - / 3 r 2 ( t )
(11)
The correlation between the number of atoms contributing to the cluster growth and the number of atoms consti-
B. Zhang, L. Zhou/Thin Solid Films 307 (1997) 21-28
26
n,aax 1t6
%
1/12 -
A
1/12
ff 0.5
I 1'
1.5
0
0.5
r. [cz/(rs~)]
(a) n~
1
1.5 (a) Nucleation on a convex surface feature
r, [c¢(r~5)] (b) n*=n,n~.v t*=to
t~
i/6
- - ~
iJ:2
I
.....
::.::.::):.
(1o) Nucleation on a plane surface feature
0
0.5
1
: .....
(c) Nucleation on a concave surface feature
Fig, 10. N u c l e a t i o n d i a n m n d on three different s u r f a c e f e a t u r e s r e p r e s e n t ing d i f f e r e n t c o n d i t i o n s o f substrate p r e t r e a t m e n t . T h e c o n v e x s u r f a c e f e a t u r e s results tn the smallest c o n t a c t angle.
1,5
t, [o:/(rl~)] (c) n*>n~ t*--,,Fig, 9. R e l a t i o n s h i p b e t w e e n the n u m b e r o f a t o m s in a critical n u c l e u s a n d the n u m b e r o f a t o m s in a c l u s t e r nrnax : (a) a g r o w i n g a t o m c l u s t e r b e c o m e a critical n u c l e u s at t = t " < to: (b) a g r o w i n g a t o m cluster n o t b e c o m e a critical n u c l e u s until t = t ~ = to: (c) an a t o m c l u s t e r n e v e r b e c o m e a critical n u c l e u s e v e n w h e n t ~ ce.
n* can can can
contrast, the substrate with the concave surface feature is not appropriate for diamond nucleation due to its large thermodynamic barrier. According to Eqs. (17) and (19), the following relationship can be obtained,
,/; < tuting a critical nucIeus is graphically shown in Fig. 9. Three different cases are presented in Fig. 9 concerning the nucleus formation. If n ~ < nmax, a growing atom cluster can become a critical nucleus when t = t* < t o which is shown in Fig. 9a; if n" = nmax, a ~ o w i n g atom cluster can not become a critical nucleus until t = t ~ = t o which is shown in Fig. 9b; if n* > t/max, the atom cluster can never become a critical nucleus even when t--+ w which is shown in Fig. 9c. The correlation between n" and n(t) not only directly determines whether a steady-state atom cluster can be converted to a critical nucleus, but also indicates how much time nucleation takes if nucleation is possible. The correlation can serve as a practical guideline for diamond coating applications. If a nucleus is located on the substrate with a convex, plane or concave surface feature as shown in Fig. 10, the contact angle between the nucleus and the substrate is represented by 0 t, 0 2, or 0 3, respectively, and has the following relationship, 0t < 02 < 03
< n;
(20)
This relationship depicts that the number of atoms required to form a critical nucleus is the smallest for the convex type substrate, whereas, it is the largest for the concave type substrate. An ideal nucleation site should be protuberant. The analytical result obtained in this study also has a good agreement with the experimental observations obtained by Dennig and Stevenson shown in Fig. 11 [1,2]. Since the most favorable surface morphology for nucleation has been proven to be convex type protuberance, the convex type morphology could be obtained by modifying the surface of a substrate using certain techniques, such as sandblasting technique in this study. The three dimensional surface morphology of the substrates used for sample groups S t and S 3 are shown in Fig. 12. The substrates in sample group S~ were ground before diamond coating. The surface skewness value of these
(18)
In referencing to Eq. (4). the thermodynamic barriers corresponding to the above different surface features can be obtained as: G;
(19)
It is, therefore, understood that the thermodynamic bartier on a convex type substrate is the lowest which facilitates an easy formation of nucleus on the substrate. In
Fig, 1 t. N u c l e a t i o n o f d i a m o n d clusters o n p y r a m i d s o f a c o n v e x type substrate [ 1,2].
B. Zhang, L. Zhou /Thin Solid Films 307 (1997) 21-28
10 gm
(a) Sandblasted sample (S3) with convex surface feature, SK=- 1.885
10grn Co) Ground sample (SI/with concave surface feature, SK=+1.203
Fig. 12. Surface morphologies of the subbtrates with and without sandblasting pretreatment: (a) sandblasted sample (S 3) with convex surface feature, S k = - 1.885; (b) ground sample (S 1) with concave surface feature, S k = + 1.203.
substrates was obtained using the Surfanalyzer 5000 (Federal Products) as S k = + 1.203 indicating a concave type surface. On the other hand, the substrates in sample g o u p S 3 were ground, and then, sandblasted with #120 and #500 alumina sands. Because of the sandblasting pretreatment, a negative skewness value was obtained on these substrates as S k = -1.885. The negative skewness value is indicative of a convex type substrate. Skewness has widely been accepted as an effective indicator of surface morphology. For an asynunetric distribution of surface heights, the skewness value may be positive or negative. A surface with a comparatively large negative skewness, e.g. Sk < - 1, usually has a good bearing property. Conversely, a comparatively large positive skewness, e.g. S k > 1, may indicate the presence of comparatively few spikes on the surface which can be the case for a concave type surface. This type of surface does not have a good bearing property and may quickly wear out when sliding against another surface. When diamond synthesis takes place on a substrate with the convex surface feature, the diamond coating approximately duplicates the configuration of the substrate surface. The enhanced nucleation density and adhesion strength of the diamond coating on the convex type substrate can increase the wear-resistance of a cutting tool.
maximum number of atoms in an atom duster prior to diamond nucleation which is a function of contact angle. The smaller the maximum number of atoms is. the easier is diamond nucleation. A steady-state atom cluster evolves into a diamond nucleus only if the number of atoms of the critical nucleus is smaller than the maximum number of atoms in the steady-state atom cluster. The number of atoms of the critical nucleus can determine the nucleation time of diamond. Among the commonly obtainable surface features, i.e., convex, plane, and concave surface features, the convex surface feature requires the smallest number of atoms for an atom cluster to evolve its diamond nucleation. Therefore, an ideal substrate should possess the convex surface feature in order to facilitate an easy nucleation, and to further obtain a high nucleation density and adhesion strength of diamond coatings. Sandblasting technique is effective in obtaining an enhanced nucleation density and adhesion strength of diamond coatings. The ideal skewness value of a substrate is between - 1 to - 3 which can be achieved by sandblasting. A better adhesion strength and wear-resistance results from the diamond coatings on substrates blasted with finer sands.
7. Nomenclature a
Ea G~
el, a~, c ; Gr at,
k in
N
6. Conclusions The effect of sandblasting on the nucleation density and adhesion strength of diamond coatings is studied both theoretically and experimentally. Based on this study, the following findings are obtained. In the process of diamond deposition, there exists a
27
1l
distance between adsorption sites activating energy of atom migration free energy required to form an atom cluster of critical size 7"* free energies required to form critical atom clusters on convex, plane and concave features free energ-y required to form an atom cluster of size r free energy change per unit volume for an atom cluster growth constant mass of a carbon atom total number of atoms deposited on substrate surface per unit time number of diffusion atoms collected by cluster per unit time number of atoms etched by atomic hydrogen per unit time number of atoms deposited per unit area per unit time number of atoms within a steady-state atom cluster number of atoms of the critical size of a nucleus
28 171' /72' t l ;
n max no
P
p(:) RH F F F O, r I Sk
T t t" to
O/
/3 Y Or
0 01, 02, 03
B. Zhang, L. Zhou / Thin Solid Films 307 (1997) 21-28
number of atoms of the critical size of a nucleus on convex, plane and concave surface features maximum number of atoms of the critical size of a nucleus number of atoms per unit volume reaction pressure distribution of surface morphology height etching rate of atom clusters due to atomic hydrogen radius of an atom cluster critical radius of a steady-state atom cluster coefficients of fLrst-order approximation of cluster radius surface skewness deposition temperature cluster growth time nucleation time growth time required to reach the maximum number of atoms in a cluster surface morphology height measured from the mean line ap(21rmkT)-1/z(2rr sin 0) 27(1 - cos O)R H surface energy standard deviation of p(z) contact angle between a cluster and substrate surface contact angles between a cluster and substrates with convex, plane and concave surface features vibration frequency of atoms deposited on substrate surface
Acknowledgements The authors gratefully acknowledge the research supports from the National Science Foundation (through the Manufacturing Processes and Equipment Program with grant No. DDM-93-9669) of the USA, the National Science Foundation of the PR China and form the University of Connecticut Research Foundation.
References [t] P.A. Dennig, D.A. Stevenson, Proceedings of the International Conference on New Diamond Science and Technology, 403 (1991). [2] P.A, Dennig, D.A. Stevenson, Proceedings of the First International Conference on the Applications of Diamond Films and Related Materials, 383 (1991). [3] K. Hirabayashi, Y. Tanaguchi, O. T~amatsu, T. Ikeda, K. Ikoma, N.I. Kurihara, J. Appl. Phys. Lett. 53 (1988) 1815. [4] P. Badzian, W.S. Verwoerd, W.P. Ellis, N.R. Greiner, Nature 343 (1990) 244. [5] M. Frenklach, R. Kematick, D. Huang, W, Howard, K.E. Spear, J. Appl. Phys. 66 (1989) 395. [6] M. Grabow, G. Gilmer, Surf. Sci. 194 (1988) 333. [7] 1.S, Ma, H. Kawarada, T. Yonehara, J.I, Suzuki, J. Wek Y. Yokota, A. Hiraki, J. Appl. Phys. Lett. 55 (1989) 1071. [8] S. Yugo, T. Kanai, T. Kimura, Diamond Rel. Mater. i (1991) 929. [9] K. Hirabayski, K. Nishimura et al., Phys. Rev. 55 (i988) 1815, [10] J. Kenneth, J. Klabunde, Thin Films from Free Atoms and Particles, Academic Press, 1985. [11] B. Zhang, S. Chen, J. Appl. Phys. 79 (1996) 9. [12] L.R.B. Elton, Introductory Nuclear Theory, Sir Isaac Pitman and Sons, London, 1959. [I3l M. Frenklach, K.E. Spear, J. Mater. Res. 3 (1988) 133. [14] Z. Xue, Q. Wu, H. Li, Thin Film Physics, Electronic Industry Press, Beijing, 1991.
ELSEVIER
Thin Solid Films 307 (1997) 21-28
J
7"7
ll/H$
Effect of sandblasting on adhesion strength of diamond coatings Bi Zhang *, Lanying Zhou Department of Mechanical Engineering, Precisioff Mam(acturing bzsfitute, Unicersit), of Connecticut, S~orrs, 06269-5119, CT, USA
Received 9 December I996: accepted 13 May i997
Abstract Premature failure of a diamond-coated tool often results from a poor adhesion of the coating and shortens the lifetime of the tool. This study presents the results of increasing the adhesion strength of diamond coatings on cutting tool inserts by pretreating the inserts with sandblasting technique to obtain a desirable surface morphology of the inserts. A geometric model representing the ideal surface morphology is established to enhance the nucleation density and adhesion strength of diamond coatings. Diamond coating experiment is conducted on the substrates of four different sample groups. Indentation and wear tests are performed on diamond-coated inserts to evaluate the effect of sandblasting on the adhesion strength of the coatings. A theoretical analysis is provided on the formation and growth of atom clusters in terms of the contact angle and the thermodynamic barrier of a substrate to predict diamond nucleation. The theoretical prediction has a good agreement with the experimental results obtained in this study and by Dennig and Stevenson [P.A. Dennig, D.A. Stevenson, Proceedings of the International Conference on New Diamond Science and Technology, 403 (1991): P.A. Dennig, D.A. Stevenson, Proceedings of the First International Conference on the Applications of Diamond Films and Related Materials, 383 (1991).]. © 1997 Elsevier Science S.A. Keywords: Adhesion; Coatings; Diamond; Surface morphology
1. Introduction The subject of diamond coating has widely been investigated for the last decade. Dennig and Stevenson studied diamond nucleation on chemically etched silicon substrates, and observed that the majority of nucleation events occurred on protruding surface features [1,2]. Other researchers used grinding and polishing techniques to enhance the nucleation density of diamond [3,4]. In a coating process, control of diamond deposits requires an extensive "knowledge of nucleation enhancement [5]. Not only are the atomic mechanisms for the growth of diamond very important [6,7], but substrate parameters such as defect type and density [8], material surface energy [9], morphology [10], crystal structure [11], and lattice constant [12] are also important, since they can dominate the nucleation behavior of diamond particles on a particular substrate. Many investigators used various techniques to modify the surfaces, and therefore, the nucleation behavior of their substrates. A commonly used method to enhance the nucleation den-
* Corresponding author. Tel.: + 1 860 4861489; fax: + 1 860 4862269; e-mail: [email protected] 0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights rese~'ed. PII S0040-6090(97)00297-6
sity and adhesion strength of diamond coatings is to scratch the surface of a substrate with diamond powders. However, not all means of surface pretreatment are appropriate for diamond coating. Scratching an optical surface with abrasives would damage that surface although it could enhance the nucleation density and growth rate of diamond [13]. The application of the scratching method is, therefore, limited. In a diamond coating process, diamond nucleation usually involves the following sequence: surface adsorption formation of steady-state atom cluster ~ cluster growth nucleus formation. Etching by atomic hydrogen may take place during cluster formation and growth. Concerning cluster growth, one may raise the following questions. At what size do the clusters become diamond nuclei? Can multi-clusters grow simultaneously? Is the process of cluster formation reversible? The aim of this study is to answer the questions through the exploration of the effect of substrate pretreatment on diamond nucleation by varying the surface geometric morphology of a substrate. A geometric model of an ideal surface morphology is proposed, and a mathematical expression representing steady-state atom cluster growth is developed based on the surface free energy of a substrate
B. Zhang, L. Zhou / Thin Solid Fihns 307 (1997) 21-28
22
to predict diamond nucleation. Diamond coating experiment is performed on the substrates pretreated with sandblasting of alumina powders. The diamond coatings deposited in the experiment are then evaluated using indentation and wear tests to verify the theoretical prediction. The substrate pretreatment shows a promising result on the enhancement of adhesion strength of diamond coatings. Fig. 2. G e o m e t r i c model representing an ideal surface m o r p h o l o g y : p y r a m i d s on hemispheres with skewness values b e t w e e n - 1 a n d - 3.
2. Surface morphology model Diamond nucleates on the surface of a substrate in a coating process. When forming a critical nucleus, it is necessary to increase the free energy of the system in which an atom cluster grows. If the nucleus is spherical with a radius r, the free-energy change can be given as 4
Gr = 4"r;rS7 + -~ ~ r 3 Q
(1)
For a nucleus with a small radius, e.g., a cluster of carbon atoms, the first term of the equation predominates. As the size of the atom cluster increases, the free energy required to form it also increases. For any further growth of the cluster, however, the second term tends to be larger than the first one, so that once the cluster reaches a certain critical size, the second term becomes dominant. Further growth of the cluster leads to a decreased free energy requirement, and thus, a steady state growth. The diamond nucleation process can be described by three steps which can be schematically shown in Fig. 1.
C
C
A & D -- platte B & E -- cc~eave C & F -- convex
The first step involves surface adsorption at which atom clusters form on the surface of a substrata at random distribution. The second step consists of cluster formation. From the surface adsorption to cluster formation, the substrata-gas phase interface with a higher free energy (e.g., surface defect, convex or concave surface feature) would favor the cluster formation, which would result in a preferential placement of the atom clusters at points B, C, E, and F as shown in Fig. 1c. The third step is nucleation step. At the nucleation step, the points with the concave surface feature will not favor the further growth of the atom clusters due to a higher free energy requirement, and therefore, the clusters shrink and eventually vanish at points B and E. Conversely, the clusters at points C and F continuously grow to form nuclei, and can grow indefinitely large until they meet each other to form diamond crystals. In general, a diamond coating can be formed uniformly and approximately duplicate the geometric configuration of a substrate surface. Since diamond nucleation is in close relationship with the surface morphology of a substrate, a mode] representing the ideal surface morphology of a substrate is constructed and shown in Fig. 2. In this model, the ideal morphology is a protuberance such as a ridge on a pyramid feature, on which a steady-state atom cluster should be formed favorably. The skewness of the ideal surface morphology can be described by the following equation,
Sk = o'-3 L+°~z3p( z)dz
(a) Subs*ratewith various surface features
(b) Atom adsurbaon on the surface features
(2)
Based on the experimental results obtained in this study, the diamond coatings with the highest nucleation density and adhesion strength were obtained on substrates with
Ptasma torch
• (d) Diamond nucleation on the convex surface feature
Window A-~ubstrate
l (b) Atom cluster formation on the surface features
Fig. 1. Schematics s h o w i n g nucleation process. O n l y the convex surface feature is ideal for d i a m o n d nucleation although atom cluster formation a n d gas adsorption can take place on the surface features.
[--~~--] CH4
H2
To pump
Ar
Fig. 3. Schematic d i a g r a m o f DC p l a s m a apparatus with tungsten a n d cerium as p l a s m a discharge electrodes, methane gas as carbon source. h y d r o g e n as reaction gas a n d argon as protective gas.
B. Zhang, L. Zhou / 77m7 Sohd Fihns 307 (!997) 21-28
Table 1 Experimental conditions for diamond coating Coating time Current Voltage Methane gas flow rate Hydrogen gas flow rate Substrate distance from electrode Substrate temperature Chamber pressure
1.5 h 102 A 30 V 25.0 sccm i580 sccm 6.0 cm 850°C 3000 Pa
pyramids of 3 - 5 /zm in diameter and 0 . 2 - 1 . 0 /xm in depth. By substituting these values into Eq. (2), the skewness value of the ideal surface morphology is then obtained as S k = - 1 - - 3.
3. Experimentation Diamond coating experiments were conducted using a DC Plasma apparatus schematically shown in Fig. 3. The plasma discharge electrode was made of tungsten and cerium. Methane gas was used as a carbon source while hydrogen as reaction gas and argon as protective gas. The experimental conditions are summarized in Table 1. The conditions for the substrate pretreatment were varied, however, the other conditions remained unchanged. The conditions for substrate pretreatment are listed in Table 2. The substrates were first degreased in acetone and ethanol, rinsed in deionized water, and then, sandblasted when necessary. After sandblasting, the substrates were suspended in an ultrasonic bath of ethanol, rinsed in deionized water, finally cleaned in H F acid, and rinsed in deionized water again. The substrate material was tungsten carbide (WC + 6% Co). Samples of four groups were used in the experiment. Sample group S o were ground without sandblasting and diamond coating. Sample group S~ were ground, and then, coated with diamond without sandblasting. Sample group S 2 were ground, blasted with # 1 2 0 alumina sands, and coated with diamond. Sample group S 3 were ground, blasted with # 1 2 0 and # 5 0 0 alumina sands, and coated with diamond. The sandblasting was performed at a pressure of 2 . 5 - 3 k g / c m a for 30 min in water carrier. The adhesion strength of the samples was evaluated using a Rockwell hardness tester, while the wear-resistance of
the samples was tested using a block-on-shaft apparatus. All the diamond coatings were characterized by scanning electron microscopy (SEM) and Raman spectroscopy.
4. Experimental results Fig. 4 shows SEM photographs of the diamond coatings on sample groups S 1 and S 3. The crystal size of the diamond coating deposited on sample group S 1 was larger than that on sample group S 3. The crystal size of sample group S 3 was approximately 3 /xm. while that of sample group S 1 was 6 p.m. Fig. 5 presents Raman spectra of the diamond coatings on sample groups $1 and S 3. No significant difference is identified between the sample groups in terms of Raman peak width at 1334 cm -1 for diamond and 1550 cm -1 for non-diamond. The indentation test results are shown in Fig. 6. At a load of 600 N, coating flaking was observed on sample group S t but not on sample group S 3. In fact, coating flaking on sample group S 3 was not observed until the indentation load was increased to 800 N. The exact failure mechanisms are to be investigated for the sample groups used in this study. However, based on the experimental observations, shear stress induced delamination may be considered as a failure mechanism for the flaking of sample group S 3, while tensile stress induced chipping as a failure mechanism for the flaking of sample group S t . The experimental results showed that the adhesion strength of a diamond coating was increased when a sample was pretreated with sandblasting than that without sandblasting.
Table 2 Tungsten carbide substrates subjected to various pretreatment conditions Sample group
Grinding
# 120 blasting
#500 blasting
Diamond coating
S0 St S2 S3
yes yes yes yes
NflA N/A yes yes
N/A N/A N/A yes
NflA yes yes yes
Fig. 4. SE~VFphotographsof diamond coatings on substrates with and without sandblasting pretreatment,
B, Zfiang, L, Zhou/Thin Solid Films 307 (1997) 21-28
24
30
2.0:
& ×
.5
10I
%
q
V=400 mTmm N=87 N Block: diamondcoated Shaft: tungstencarbide Lubricant: 20# engine oil
25
2o
_
Block,
i!/
$N
o,5 ¢
Shat~
~.
&0 1100
I
I
1
I
I
1200
1300
1400
1500
1600
10
V
1700
Raman shift, cm'l
(a) Sandblasted sample from group g3
n S3
2.0
S,_
St
So
Sample group
x
~.5
0.5
i1 /
Fig. 7, Block-on-shaft wear rate of various samples subjected to respective pretreatment conditions. A decreasing wear rate is observed from the samples pretreated by sandblasting with fine and coarse sands and by grinding.
~'~
0,0
1100
I
I
I
I
I
1200
1300
1400
t 50fl
160i)
1700
Raman shift, cm-1
In addition, the blasting pretreatment with finer sands can provide an enhanced wear-resistance than that with coarser sands.
Co) Ground sample from group S~ Fig. 5. Raman spectra of the diamond coatings, No significant difference identified between the two sample groups.
The wear-resistance test results of different sample groups are shown in Fig. 7. It was found that the wear rate of sample group S 3 was 14 times lower than that of sample group So; 5.5 times lower than that of sample group St: and 2 times lower than that of sample group S 2. The experimental results revealed that the wear-resistance of a diamond coating deposited on a substrate pretreated with sandblasting is much higher than that without sandblasting.
5. D i s c u s s i o n
The effect of the sandblasting pretreatment of substrate on the adhesion strength of diamond coatings has been demonstrated by the experiments. The growth of a polycrystalline diamond film begins, after some incubation time, with surface adsorption on a substrate, which leads to the formation of atom clusters. The free energy necessary to form an atom cluster increases with the increase of the size of the atom cluster. Once a steady-state atom cluster reaches a certain critical size, the corresponding critical free energy is the thermodynamic barrier for nucleation on a substrate. The critical size of the atom cluster and the thermodynamic barrier are expressed by the following equations: -2y =
- -
(3)
16~f(O)~/3 3G~
(4)
where (2 + cos 0 ) ( 1 - cos 0) 2
f(0) =
Fig. 6. Effect of sandblasting on indentation strength of diamond coatings.
4
(5)
An atom cluster having a radius less than the critical size is called a subcritical embryo; and those of the critical size or larger are called critical and supercritical nuclei. The thermodynamic barrier is a function of the surface free energy of the substrate at nucleating sites and contact angle between a nucleus and the substrate. A general trend
B. Zhang, L. Zhou / Thin Solid Fihns 307 (1997) 21-28
of such a nucleating site is to reduce the thermodynamic barrier for nucleation. When a nucleus forms on a substrate, a high energy substrate-gas phase interface is replaced by a lower energy substrate-nucleus interface, thereby, resulting in a smaller overall surface energy contribution. Nucleation would begin at a high energy center, e.g. a defect site, of a substrate. But a different type of high energy centers would result in a different substratenucleus contact angle. The thermodynamic barrier for nucleation decreases with decreasing contact angle and approaches zero as the contact angle approaches zero. The decrease of the thermodynamic barrier facilitates an easy growth of the atom clusters, and thus, the formation of nuclei. A lower thermodynamic bmTier can be obtained from a substrate of the convex surface feature because such a substrate provides a small contact angle for nucleation. Besides, the probability for the atom cluster to convert to a nucleus onto such a surface is much higher than that onto other types of surfaces due to the same reason. Whether an atom cluster can grow to become a nucleus depends on the rates of atom deposition and atom etching. The number of diffusion atoms deposited onto a cluster in a unit time can be written as: Nd
=
( 2 e r r sin O)Noavse -zd/kT
(6)
The etching rate of the atom cluster due to atomic hydrogen is directly proportional to the surface area of the steady-state atom cluster. The number of atoms etched by atomic hydrogen in a unit time is: N H = 2~-r2(1 - cos O)R u
(7)
so the number of atoms contributing to the cluster growth in a unit time is then obtained as:
N=Noav~e-E~/kr(2rrr sin 0) - 2~-r2(1 - cos O)R H
(s) N Ou~e -ed/kr = p(2rrmkr) - t / z
&-a~ 1/6
ff
1/i2
o5
Fig. 8. G r o w t h cmwe for a steady-state a t o m d u s t e r : at t o = a / ( r I ~), the n u m b e r o f atoms in a cluster reaches the m a x i m u m .
Through the first-order approximation, the radius of an atom cluster can be expressed as:
r(t) = r o + rlt
N is then represented by
N = ap(Z~rmkT) -1/22rrr sin 0 - 2~rr2(1 - cos O) R H
N(t) =
(13)
At moment t, the number of atoms within the steadystate atom cluster is given by:
,2(t) = fo~N( t)dt
a = ap(2crmkT)-1/2(2rr sin 0) and /3 = 2~r(1 - cos O)R H
(14)
By substituting Eq. (13) into Eq. (14), the number of atoms contributing to the cluster growth is given as:
n(t) = ---/3r?t3 + 3
0~1"1
2
t2 + C
According to the initial condition of the cluster growth, n(0) = 0 at t = 0, the inte~ation constant is found to be C = 0. Eq. (14) can, therefore, be rewritten as:
~3r--t3 +
C~t'l /,2
(15)
2
Eq. (15) represents the number of atoms contributing to the duster growth in terms of deposition time, which is also plotted in Fig. 8. The maximum number of atoms in a cluster is then found at t o = a / ( / 3 r 1) as:
(10) The atom cluster can grow if N is greater than zero. Otherwise, it may shrink back and vanish due to the etching effect by atomic hydrogen. Let
(12)
with an assumption that the radius is zero when at time zero under the initial conditions, Eq. (I 1) is changed to
3 (9)
1.5
t, [d(~,5):
n(t)
which is also evidenced by Elton [12]. Since
25
[ ap(2 ¢rmkT)- 1/2( 2 73" sin 0 )]3 '/max =
6rl[2~r(1 - cos 0 ) R . ] :
(t6)
Since the thermodynamic barrier is a measure of the free energy required to form a critical nucleus, its value varies with many factors, such as the contact angle between an atom cluster and substrate surface. The number of atoms of a critical nucleus is [14] n* = no e - c • / k r
(17)
then the following equation can be obtained
N( t) = a r ( t ) - / 3 r 2 ( t )
(11)
The correlation between the number of atoms contributing to the cluster growth and the number of atoms consti-
B. Zhang, L. Zhou/Thin Solid Films 307 (1997) 21-28
26
n,aax 1t6
%
1/12 -
A
1/12
ff 0.5
I 1'
1.5
0
0.5
r. [cz/(rs~)]
(a) n~
1
1.5 (a) Nucleation on a convex surface feature
r, [c¢(r~5)] (b) n*=n,n~.v t*=to
t~
i/6
- - ~
iJ:2
I
.....
::.::.::):.
(1o) Nucleation on a plane surface feature
0
0.5
1
: .....
(c) Nucleation on a concave surface feature
Fig, 10. N u c l e a t i o n d i a n m n d on three different s u r f a c e f e a t u r e s r e p r e s e n t ing d i f f e r e n t c o n d i t i o n s o f substrate p r e t r e a t m e n t . T h e c o n v e x s u r f a c e f e a t u r e s results tn the smallest c o n t a c t angle.
1,5
t, [o:/(rl~)] (c) n*>n~ t*--,,Fig, 9. R e l a t i o n s h i p b e t w e e n the n u m b e r o f a t o m s in a critical n u c l e u s a n d the n u m b e r o f a t o m s in a c l u s t e r nrnax : (a) a g r o w i n g a t o m c l u s t e r b e c o m e a critical n u c l e u s at t = t " < to: (b) a g r o w i n g a t o m cluster n o t b e c o m e a critical n u c l e u s until t = t ~ = to: (c) an a t o m c l u s t e r n e v e r b e c o m e a critical n u c l e u s e v e n w h e n t ~ ce.
n* can can can
contrast, the substrate with the concave surface feature is not appropriate for diamond nucleation due to its large thermodynamic barrier. According to Eqs. (17) and (19), the following relationship can be obtained,
,/; < tuting a critical nucIeus is graphically shown in Fig. 9. Three different cases are presented in Fig. 9 concerning the nucleus formation. If n ~ < nmax, a growing atom cluster can become a critical nucleus when t = t* < t o which is shown in Fig. 9a; if n" = nmax, a ~ o w i n g atom cluster can not become a critical nucleus until t = t ~ = t o which is shown in Fig. 9b; if n* > t/max, the atom cluster can never become a critical nucleus even when t--+ w which is shown in Fig. 9c. The correlation between n" and n(t) not only directly determines whether a steady-state atom cluster can be converted to a critical nucleus, but also indicates how much time nucleation takes if nucleation is possible. The correlation can serve as a practical guideline for diamond coating applications. If a nucleus is located on the substrate with a convex, plane or concave surface feature as shown in Fig. 10, the contact angle between the nucleus and the substrate is represented by 0 t, 0 2, or 0 3, respectively, and has the following relationship, 0t < 02 < 03
< n;
(20)
This relationship depicts that the number of atoms required to form a critical nucleus is the smallest for the convex type substrate, whereas, it is the largest for the concave type substrate. An ideal nucleation site should be protuberant. The analytical result obtained in this study also has a good agreement with the experimental observations obtained by Dennig and Stevenson shown in Fig. 11 [1,2]. Since the most favorable surface morphology for nucleation has been proven to be convex type protuberance, the convex type morphology could be obtained by modifying the surface of a substrate using certain techniques, such as sandblasting technique in this study. The three dimensional surface morphology of the substrates used for sample groups S t and S 3 are shown in Fig. 12. The substrates in sample group S~ were ground before diamond coating. The surface skewness value of these
(18)
In referencing to Eq. (4). the thermodynamic barriers corresponding to the above different surface features can be obtained as: G;
(19)
It is, therefore, understood that the thermodynamic bartier on a convex type substrate is the lowest which facilitates an easy formation of nucleus on the substrate. In
Fig, 1 t. N u c l e a t i o n o f d i a m o n d clusters o n p y r a m i d s o f a c o n v e x type substrate [ 1,2].
B. Zhang, L. Zhou /Thin Solid Films 307 (1997) 21-28
10 gm
(a) Sandblasted sample (S3) with convex surface feature, SK=- 1.885
10grn Co) Ground sample (SI/with concave surface feature, SK=+1.203
Fig. 12. Surface morphologies of the subbtrates with and without sandblasting pretreatment: (a) sandblasted sample (S 3) with convex surface feature, S k = - 1.885; (b) ground sample (S 1) with concave surface feature, S k = + 1.203.
substrates was obtained using the Surfanalyzer 5000 (Federal Products) as S k = + 1.203 indicating a concave type surface. On the other hand, the substrates in sample g o u p S 3 were ground, and then, sandblasted with #120 and #500 alumina sands. Because of the sandblasting pretreatment, a negative skewness value was obtained on these substrates as S k = -1.885. The negative skewness value is indicative of a convex type substrate. Skewness has widely been accepted as an effective indicator of surface morphology. For an asynunetric distribution of surface heights, the skewness value may be positive or negative. A surface with a comparatively large negative skewness, e.g. Sk < - 1, usually has a good bearing property. Conversely, a comparatively large positive skewness, e.g. S k > 1, may indicate the presence of comparatively few spikes on the surface which can be the case for a concave type surface. This type of surface does not have a good bearing property and may quickly wear out when sliding against another surface. When diamond synthesis takes place on a substrate with the convex surface feature, the diamond coating approximately duplicates the configuration of the substrate surface. The enhanced nucleation density and adhesion strength of the diamond coating on the convex type substrate can increase the wear-resistance of a cutting tool.
maximum number of atoms in an atom duster prior to diamond nucleation which is a function of contact angle. The smaller the maximum number of atoms is. the easier is diamond nucleation. A steady-state atom cluster evolves into a diamond nucleus only if the number of atoms of the critical nucleus is smaller than the maximum number of atoms in the steady-state atom cluster. The number of atoms of the critical nucleus can determine the nucleation time of diamond. Among the commonly obtainable surface features, i.e., convex, plane, and concave surface features, the convex surface feature requires the smallest number of atoms for an atom cluster to evolve its diamond nucleation. Therefore, an ideal substrate should possess the convex surface feature in order to facilitate an easy nucleation, and to further obtain a high nucleation density and adhesion strength of diamond coatings. Sandblasting technique is effective in obtaining an enhanced nucleation density and adhesion strength of diamond coatings. The ideal skewness value of a substrate is between - 1 to - 3 which can be achieved by sandblasting. A better adhesion strength and wear-resistance results from the diamond coatings on substrates blasted with finer sands.
7. Nomenclature a
Ea G~
el, a~, c ; Gr at,
k in
N
6. Conclusions The effect of sandblasting on the nucleation density and adhesion strength of diamond coatings is studied both theoretically and experimentally. Based on this study, the following findings are obtained. In the process of diamond deposition, there exists a
27
1l
distance between adsorption sites activating energy of atom migration free energy required to form an atom cluster of critical size 7"* free energies required to form critical atom clusters on convex, plane and concave features free energ-y required to form an atom cluster of size r free energy change per unit volume for an atom cluster growth constant mass of a carbon atom total number of atoms deposited on substrate surface per unit time number of diffusion atoms collected by cluster per unit time number of atoms etched by atomic hydrogen per unit time number of atoms deposited per unit area per unit time number of atoms within a steady-state atom cluster number of atoms of the critical size of a nucleus
28 171' /72' t l ;
n max no
P
p(:) RH F F F O, r I Sk
T t t" to
O/
/3 Y Or
0 01, 02, 03
B. Zhang, L. Zhou / Thin Solid Films 307 (1997) 21-28
number of atoms of the critical size of a nucleus on convex, plane and concave surface features maximum number of atoms of the critical size of a nucleus number of atoms per unit volume reaction pressure distribution of surface morphology height etching rate of atom clusters due to atomic hydrogen radius of an atom cluster critical radius of a steady-state atom cluster coefficients of fLrst-order approximation of cluster radius surface skewness deposition temperature cluster growth time nucleation time growth time required to reach the maximum number of atoms in a cluster surface morphology height measured from the mean line ap(21rmkT)-1/z(2rr sin 0) 27(1 - cos O)R H surface energy standard deviation of p(z) contact angle between a cluster and substrate surface contact angles between a cluster and substrates with convex, plane and concave surface features vibration frequency of atoms deposited on substrate surface
Acknowledgements The authors gratefully acknowledge the research supports from the National Science Foundation (through the Manufacturing Processes and Equipment Program with grant No. DDM-93-9669) of the USA, the National Science Foundation of the PR China and form the University of Connecticut Research Foundation.
References [t] P.A. Dennig, D.A. Stevenson, Proceedings of the International Conference on New Diamond Science and Technology, 403 (1991). [2] P.A, Dennig, D.A. Stevenson, Proceedings of the First International Conference on the Applications of Diamond Films and Related Materials, 383 (1991). [3] K. Hirabayashi, Y. Tanaguchi, O. T~amatsu, T. Ikeda, K. Ikoma, N.I. Kurihara, J. Appl. Phys. Lett. 53 (1988) 1815. [4] P. Badzian, W.S. Verwoerd, W.P. Ellis, N.R. Greiner, Nature 343 (1990) 244. [5] M. Frenklach, R. Kematick, D. Huang, W, Howard, K.E. Spear, J. Appl. Phys. 66 (1989) 395. [6] M. Grabow, G. Gilmer, Surf. Sci. 194 (1988) 333. [7] 1.S, Ma, H. Kawarada, T. Yonehara, J.I, Suzuki, J. Wek Y. Yokota, A. Hiraki, J. Appl. Phys. Lett. 55 (1989) 1071. [8] S. Yugo, T. Kanai, T. Kimura, Diamond Rel. Mater. i (1991) 929. [9] K. Hirabayski, K. Nishimura et al., Phys. Rev. 55 (i988) 1815, [10] J. Kenneth, J. Klabunde, Thin Films from Free Atoms and Particles, Academic Press, 1985. [11] B. Zhang, S. Chen, J. Appl. Phys. 79 (1996) 9. [12] L.R.B. Elton, Introductory Nuclear Theory, Sir Isaac Pitman and Sons, London, 1959. [I3l M. Frenklach, K.E. Spear, J. Mater. Res. 3 (1988) 133. [14] Z. Xue, Q. Wu, H. Li, Thin Film Physics, Electronic Industry Press, Beijing, 1991.