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The effect of phosphorus content and heat treatment on the wear resistance of electroless deposited Ni-P alloys
Wear, 103 (1985)
269
269 - 278
THE EFFECT OF PHOSPHORUS CONTENT AND IIEAT TREA~ENT ON THE WEAR RESISTANCE OF ELEGTROLESS DEPOSITED Ni-P ALLOYS* GUANG-XI LU, GENG-FU LI and FU-CHANG YU ~epurt~ent fChino)
of ~ater~aZs ~nginee~~g,
J&n Institute
of engineering,
Changchu~
130012
(Received March 29, 1985; accepted May 9, 1985)
Summary Five Ni-P alloys with phosphorus contents ranging from 7% to 13% were prepared by electroless deposition onto steel substrates and annealing at various temperatures between 473 and 973 K for 1 h. The structures as well as the hardness and wear resistance of the alloys were studied, and the results show that as the annealing temperature is increased, the hardness and the wear resistance go through maximum values, but the two maxima do not occur at the same annealing temperature. It is suggested that the wear of the Ni-P alloys can be closely related to their structures and to the crack propagation which occurs during the wear process. 1. Introduction Electroless deposited Ni-P alloys exhibit superior corrosion resistance and good wearability. There are already some review papers and patents in the literature [l - 51 concerning the deposition processes and the corrosion resistance of the deposits. As for the relationship between the phosphorus content, the heat treatment and the wear resistance, several papers have been published since the 1960s [6 - lo], but none of them drew close connections with the structural changes which occurred during the heat treatments. In the present paper the recent work by the authors on this problem is presented, with an attempt to give an explanation of the relationship between the microstructure and the wear resistance of these alloys. 2. Experimental details Five alloys with different phosphorus contents were prepared by electroless deposition onto medium carbon steel substrates. The deposition *Paper presented at the International Canada, April 14 - l&3,1985.
Conference on Wear of Materials, Vancouver,
0043-1648/85/$3.30
0 Elsevier Sequoia/Printed
in The Netherlands
270
parameters used and the phosphorus contents of the alloys obtained are shown in Table 1. The phosphorus contents of the specimens were determined by volumetric analysis. A tube furnace was used for the heat treatment of the specimens, and the annealing temperatures employed were between 473 and 973 K, with intervals of 50 K. After the specimens had been heated for 1 h they were air cooled. To investigate the structures of the deposited alloys before and after heat treatment, X-ray diffraction and optical and electron microscopy were employed. The X-ray diffractometer used was a Rigaku Geigerflex D/maxII unit (copper target, nickel filter, 35 kV, 20 mA). Hardness measurements were made using a Knoop microhardness tester of the Frankotest 537 type, and the load used was 50 gf. Finally the wear resistances of the specimens were compared on a Skoda-Savin-type wear testing machine (rotating disc and plate type). The hard metal disc was rotated at a speed of 675 rev min- ‘, the load was 10 kgf, the total number of revolutions was 3000 and a 0.5% aqueous solution of potassium bichromate was used as a coolant. The depths of the wear impressions did not exceed half the thickness of the deposited layer.
3. Results and discussion 3.1. Structures of alloys in the as-deposited condition Figure 1 shows the X-ray diffraction patterns of the five experimental alloys in the as-deposited condition, from which we can see that the alloys with phosphorus contents of 9.61%, 10.38%, 12.1% and 13.56% all showed a wide diffraction intensity peak at about 20 = 45”. On both sides of the peak the diffraction intensity weakens rather gradually. This kind of wide intensity peak is characteristic of amorphous solids [ll] and is the result of there being no long-range-ordered structure in the amorphous state. TABLE 1 Deposition parameters and phosphorus contents of the specimens 2
3
4
-
15 14 13 6.18 348 8 9.61
25 -
-
30 10 10 -
1 Composition of solution (g 1-l) NiS04 * 7H20 NiC12*6HzO NaHzP02*H20 CH3COONa. 3Hz0 NaCsH50*Hz0 pH value Temperature (K) Duration (h) P content of specimens (%)
6.09 346 10 7.67
21 10 10 4.7 353 8 10.38
30 10 10 4.18 358 8 12.1
5
25 21 10 10 4.18 363 8 13.56
271
13.5613
35
40
45 28,
50
55
degrees
Fig. 1. X-ray diffraction patterns of the five experimental dition.
alloys in the as-deposited con-
The diffraction pattern of the 7.67% P alloy can be regarded as a wide intensity peak superimposed with a (111) diffraction peak of nickel, showing that this alloy has a mixed structure composed of an amorphous phase and crystalline nickel. The results mentioned above agree well with those already published [l, 12, 131, i.e. electroless or electrodeposited Ni-P alloys will be amorphous in structure provided that the phosphorus content of the alloy exceeds 8%. 3.2. Structural transformations during heat treatment Amorphous alloys are highly disordered in structure and are metastable. When heated at a relatively high temperature, crystallization reactions will occur, leading to changes in the alloy properties [ 141. According to the Ni-P binary phase diagram [ 151, the eutectic composition is 10 wt.% P and the two phases composing the eutectic structure are nickel (solid solution) and NisP. Therefore, of the alloys used in this investigation, one is eutectic, two are hypo-eutectic and two are hypereutectic. In Figs. 2 - 5, X-ray diffraction patterns for four experimental alloys after annealing for 1 h at different temperatures are shown. It can be seen that for hypo-eutectic alloys, nickel crystals precipitate first from the amorphous parent phase and the simultaneous crystallization of nickel and NisP then follows. The reaction is completed at about 673 K. In hypereutectic alloys, Ni$ precipitates first instead of nickel, while in the eutectic alloy only the simultaneous precipitation of nickel and NiaP is observed. The crystallization process is complete at about 723 K for both the eutectic and the hypereutectic alloys. From the crystallization process cited above, it seems that the amorphous alloys crystallize in a manner similar to the solidification of the corresponding liquid alloys. However, since the crystallization of the amorphous alloys is developed under extremely large supercoolings, the driving force for the transformation is large and the diffusivity of the atoms
272
973K
973K 873K
773K
773K 723K 673K
* .z z 9 2
723K 673K 623K
623K
573K
573K 523K
w
35
473K 35
40 29,
degrees
40 45 ~6, degrees
Fig. 2. X-ray diffraction patterns of the Ni -7.67%P peratures: X, nickel; A, NisP.
alloy after annealing at various tem-
Fig. 3. X-ray diffraction patterns of the Ni -9.61%P peratures: X, nickel; a, Ni3P.
alloy after annealing at various tem-
973K
?.
723I’
.Y
20
673x
2
35
40 26,
45
50
55
-5 35 26,
degrees
Fig. 4. X-ray diffraction patterns peratures: X, nickel; A, Ni3P.
of the Ni-10.38%P
Fig. 5. X-ray diffraction patterns of the Ni-13.56%P peratures: X, nickel; A, NiaP.
degrees
alloy after annealing at various
tem-
alloy after annealing at various tem-
273
is small, it is reasonable that the morphology of the microstructures obtained is quite different from that of the solidification structures [16]. If further annealing is applied after the completion of the crystallization process, the nickel phase will undergo an Ostwald ripening process, thus causing the structure of all the experimental alloys to consist of finely dispersed granular nickel particles embedded in an Ni,P matrix, as shown in Fig. 6.
(4
(b)
(cl Fig. 6. Microstructures of the Ni-lS.l%P alloy after annealing at various temperatures: (a) 873 K; (b) 973 K; (c) 973 K (carbon replica). The specimens were etched electrolytically in 0.5% chromic acid. (Magnifications: (a) 800x;(h) 800X; (c) 13 000x.)
3.3. Effect of 1 h annealing temperature on hardness Figure 7 shows the dependence of the hardness on the 1 h annealing temperature of the experimental alloys. The hardness changes very little with increasing temperature when the annealing temperature is below 523 K, but above that it increases abruptly. All the alloys attain maximum hardness values after heating at a particular temperature; the Ni-13.56%P alloy at about 723 K and the others at about 673 K. As the annealing temperature is further raised, the hardness values fall gradually. From the aforementioned structural changes during crystallization, it can be seen that the
274
800 -
473
673
Annealing Temp.,
873
K
Fig. 7. The dependence of the hardness on the annealing temperature for the five experimental alloys: 0, Ni-7.67%P; *, Ni-9.61%P; 0, Ni-10.38%P; 0, NC12.1%P; X, Ni13.56%P.
marked increase in hardness is due to the formation of the hard intermediate phase Ni& the hardness peaks roughly correspond to the completion of crystallization. The higher the phosphorus content, the greater the NiQ phase content and consequently the higher the peak hardness. The decrease in hardness at higher annealing temperatures can be attributed to the Ostwald ripening process of the nickel particles. 3.4. Wear resistance of electroless deposited Ni-P alloys 3.4.1. Annealing temperature versus wear resistance The relationship between wear volume and annealing temperature is illustrated in Fig. 8. All five alloys show minimum values at a particular annealing temperature; four alloys at about 873 K and the Ni-7.67%P alloy at about 723‘K. Early concepts considered that the wear resistance of metals varies linearly with their hardness. In the present work, however, except for the partially amorphous Ni-7.67%P alloy, the wear resistances of all the other four alloys do not vary in proportion with their hardnesses, i.e. the best wear resistance is not in accord with the greatest hardness. The surface morphology of the wear scar of the crystallized specimens was examined under a scanning electron microscope and the typical appearance is shown in Fig. 9. The wear tracks shown demonstrate that the
275
20 -
0 273
I 673
I 473 Annealing
Temp.,
I a73
1 73
K
Fig. 8. The dependence of the wear volume on the annealing temperature for the five experimental alloys: 0, Ni-7,67%P; A, Ni-961%P; 0, Ni-10.38%P; 0, Ni-lP.l%P; X , Ni-13.56%P.
(4
(b)
Fig. 9. Typical surface morphology of the wear scar of the Ni-9.61%P alloy annealed at 873 K (the wear direction is from left to right): SEM micrographs (secondary electron images). (Magnifications: (a) 55x; (b) 165X.)
nature of the wear involved in the test procedure is fund~en~ly abrasive. The microscopically protruded carbide particles on the hard metal disc can act as abrasive particles. Owing to the extreme brittleness of the crystallized Ni-P alloys, it is thought that some microsp~~ng may occur; which increases during further wear, thus forming the large deep tracks (white area).
276
Recent researchers have pointed out that the wear resistance of metals is dependent on the hardness, the fracture toughness and the microstructure [17,18]. This is reasonable because in general, whenever a wear debris particle is detached from the metal, the processes of elastic deformation, plastic deformation and fracture must occur, with fracture as the final and necessary process. Thus crack propagation in certain microstructures must be taken into account when the formation of wear debris is considered, particularly when the ratio of fracture toughness to hardness of the material is low [ 191. For the alloys used in this investigation, even with the alloy of lowest phosphorus content (7.67%), there would be roughly 50% of the NisP phase in the crystallized state as estimated from the phase diagram by the lever rule. For the other four alloys with higher phosphorus contents there must be an even greater NisP phase content in the crystallized structure, e.g. the estimated NisP content for the 12.1% P alloy is roughly 80%. Therefore the ductile nickel phase must be in the form of isolated particles surrounded by a brittle NisP matrix. Evidently, this sort of structure must be hard and brittle, and the toughness-to-hardness ratio is low. In fact, when the deposited layer is peeled off from the substrate and then crystallized, it is so brittle that it is hard to maintain its completeness without cracking. During the wear process, when a crack in the Ni$ matrix propagates into a nickel particle, the ductile nickel phase will undergo plastic deformation which will blunt the crack tip, relax the stress concentration at the front of the crack and slow down the propagation. If the annealing temperature is low, crystallization cannot be completed and the structure of the alloy is partially amorphous. Because the amorphous phase is tougher, the wear resistance is substantially controlled by the hardness and increases as the hardness increases. After crystallization, the nickel particles agglomerate and become larger. When a crack propagates into such a particle, enough plastic deformation can occur to slow down the growth of the crack and the wear volume can be decreased. If the particle is small, as is the case when crystallization has just finished or when agglomeration of the nickel particles is not far from starting, plastic deformation is restricted to a limited volume and stress concentration can only be relaxed to a limited degree before the crack penetrates through the particle. If the annealing temperature is still higher, the size of the nickel particles further increases, and the average distance between adjacent particles is also increased, so the mean free path for crack propagation becomes larger. This means that when a crack propagates into the matrix, the probability that it encounters a nickel particle is smaller, and so it follows that the wear volume increases and goes through a minimum. 3.4.2. Effect of phosphorus content on wear resistance The data obtained were replotted for the dependence of the wear volume on the phosphorus content, and are shown in Fig. 10 for the asdeposited and annealed (673 and 873 K) conditions. In the as-deposited
277 100
80
0 6
8
10
12
14
P, wt.% Fig. 10. The dependence of the wear volume on the phosphorus content of the five experimental alloys: o, as plated; A, annealed at 673 K; l, annealed at 873 K.
condition all the alloys have similar hardnesses, but the wear volumes decrease monotonically with increasing phosphorus content. When the annealing temperature is 673 K, most of the alloys reach their maximum hardness and the wear volumes are nearly equal. If the annealing temperature is 873 K, the alloys reach a state of best wear resistance, and the three alloys with phosphorus contents of 9.61%, 10.38% and 12.1% produce the least wear volumes. This is possibly due to the fact that at higher phosphorus contents the concentration of ductile nickel particles is so small that they are only rarely encountered by the propagating crack during the wear process. To summarize, it seems that there exists an optimum structure for best wear resistance in the Ni-P alloys, i.e. an optimum amount and size of granular nickel particles dispersed in an NisP matrix. 4. Conclusions (1) After crystallization, all amorphous Ni-P alloys have a structure of granular nickel particles embedded in an Ni$ matrix, although their transformation characteristics differ during the heat treatment. The higher the phosphorus content, the greater the Ni$ phase content. (2) The unusual increase in hardness after heat treatment is the result of crystallization. If the alloy is completely amorphous in the as-deposited
condition, the maximum hardness is reached after annealing at 673 - 723 K for 1 h. (3) Alloys with phosphorus contents of 9% - 13% which are annealed at about 873 K exhibit the best wear resistance, but this does not coincide with the highest hardness; this can be related to the structure of the alloys.
References 1 L. F. Spencer, Met. Finish., (10) (1974) 35 - 38; (11) (1974) 50 - 54; (12) (1974) 58 - 64 ; (1) (1975) 38 - 44. 2 D. W. Baudrand, Plot. Surf, Finish., 70 (12) (1983) 24 - 26. 3 P. TaImey and W. J. Crehan, U.S. Patent 2,658,839 (1953). 4 G. Gutzeit and A. Krieg, U.S. Patent 2,658,841 (1953). 5 P. TaImey and G. Gutzeit, U.S. Patent 2,762,723 (1956). 6 J. P. Randin and H. E. Hintermann, Plating (East Orange, NJ), 54 (1967) 523 - 532. 7 K. Parker, Plating (East Orange, NJ), 61 (1974) 834 - 841. 8 N. A. Tope, E. A. Baker and B. C. Jackson, flat. Surf. Finish., 63 (10) (1976) 30 - 37. 9 Teaching Group of Machine Repairing, Henan Agricultural Institute, Frict. Wear, (4) (1980) 17 - 20 (in Chinese). 10 T. Duan, Frict. Wear, (4) (1981) 37 - 41 (in Chinese). 11 G. Li, W. Liu and G. Lu, Phys. Znsp. Chem. Anal., Phys. Znsp., (6) (1982) 25 - 27 (in Chinese). 12 K. Masui, M. Shigeo and T. Yamada, J. Jpn. Inst. Met., 41 (1977) 1130 - 1134 (in Japanese). 13 G. Lu, W. Liu, G. Li and F. Yu, An investigation on the electro and electroless deposited Ni-P alloys, Res. Rep. 82336.1, December 1982 (Jilin Institute of Engineering, Changchun, China) (in Chinese). 14 M. G. Scott, Met. Technol., 7 (1980) 133 - 141. 15 M. Hansen, Constitution of&nary AZZoys, McGraw-Hill, New York, 1958, p. 1027. 16 W. Liu, G. Li and G. Lu, Trans. Met. Heat Treat., 2 (2) (1981) 57 - 64 (in Chinese). 17 E. Hornbogen, Wear, 33 (1975) 251 - 259. 18 K. H. Zum Gahr, 2. Metallkd., 69 (1978) 643 - 650. 19 M. A. Moore and F. S. King, Wear, 60 (1980) 123 - 140.
269
269 - 278
THE EFFECT OF PHOSPHORUS CONTENT AND IIEAT TREA~ENT ON THE WEAR RESISTANCE OF ELEGTROLESS DEPOSITED Ni-P ALLOYS* GUANG-XI LU, GENG-FU LI and FU-CHANG YU ~epurt~ent fChino)
of ~ater~aZs ~nginee~~g,
J&n Institute
of engineering,
Changchu~
130012
(Received March 29, 1985; accepted May 9, 1985)
Summary Five Ni-P alloys with phosphorus contents ranging from 7% to 13% were prepared by electroless deposition onto steel substrates and annealing at various temperatures between 473 and 973 K for 1 h. The structures as well as the hardness and wear resistance of the alloys were studied, and the results show that as the annealing temperature is increased, the hardness and the wear resistance go through maximum values, but the two maxima do not occur at the same annealing temperature. It is suggested that the wear of the Ni-P alloys can be closely related to their structures and to the crack propagation which occurs during the wear process. 1. Introduction Electroless deposited Ni-P alloys exhibit superior corrosion resistance and good wearability. There are already some review papers and patents in the literature [l - 51 concerning the deposition processes and the corrosion resistance of the deposits. As for the relationship between the phosphorus content, the heat treatment and the wear resistance, several papers have been published since the 1960s [6 - lo], but none of them drew close connections with the structural changes which occurred during the heat treatments. In the present paper the recent work by the authors on this problem is presented, with an attempt to give an explanation of the relationship between the microstructure and the wear resistance of these alloys. 2. Experimental details Five alloys with different phosphorus contents were prepared by electroless deposition onto medium carbon steel substrates. The deposition *Paper presented at the International Canada, April 14 - l&3,1985.
Conference on Wear of Materials, Vancouver,
0043-1648/85/$3.30
0 Elsevier Sequoia/Printed
in The Netherlands
270
parameters used and the phosphorus contents of the alloys obtained are shown in Table 1. The phosphorus contents of the specimens were determined by volumetric analysis. A tube furnace was used for the heat treatment of the specimens, and the annealing temperatures employed were between 473 and 973 K, with intervals of 50 K. After the specimens had been heated for 1 h they were air cooled. To investigate the structures of the deposited alloys before and after heat treatment, X-ray diffraction and optical and electron microscopy were employed. The X-ray diffractometer used was a Rigaku Geigerflex D/maxII unit (copper target, nickel filter, 35 kV, 20 mA). Hardness measurements were made using a Knoop microhardness tester of the Frankotest 537 type, and the load used was 50 gf. Finally the wear resistances of the specimens were compared on a Skoda-Savin-type wear testing machine (rotating disc and plate type). The hard metal disc was rotated at a speed of 675 rev min- ‘, the load was 10 kgf, the total number of revolutions was 3000 and a 0.5% aqueous solution of potassium bichromate was used as a coolant. The depths of the wear impressions did not exceed half the thickness of the deposited layer.
3. Results and discussion 3.1. Structures of alloys in the as-deposited condition Figure 1 shows the X-ray diffraction patterns of the five experimental alloys in the as-deposited condition, from which we can see that the alloys with phosphorus contents of 9.61%, 10.38%, 12.1% and 13.56% all showed a wide diffraction intensity peak at about 20 = 45”. On both sides of the peak the diffraction intensity weakens rather gradually. This kind of wide intensity peak is characteristic of amorphous solids [ll] and is the result of there being no long-range-ordered structure in the amorphous state. TABLE 1 Deposition parameters and phosphorus contents of the specimens 2
3
4
-
15 14 13 6.18 348 8 9.61
25 -
-
30 10 10 -
1 Composition of solution (g 1-l) NiS04 * 7H20 NiC12*6HzO NaHzP02*H20 CH3COONa. 3Hz0 NaCsH50*Hz0 pH value Temperature (K) Duration (h) P content of specimens (%)
6.09 346 10 7.67
21 10 10 4.7 353 8 10.38
30 10 10 4.18 358 8 12.1
5
25 21 10 10 4.18 363 8 13.56
271
13.5613
35
40
45 28,
50
55
degrees
Fig. 1. X-ray diffraction patterns of the five experimental dition.
alloys in the as-deposited con-
The diffraction pattern of the 7.67% P alloy can be regarded as a wide intensity peak superimposed with a (111) diffraction peak of nickel, showing that this alloy has a mixed structure composed of an amorphous phase and crystalline nickel. The results mentioned above agree well with those already published [l, 12, 131, i.e. electroless or electrodeposited Ni-P alloys will be amorphous in structure provided that the phosphorus content of the alloy exceeds 8%. 3.2. Structural transformations during heat treatment Amorphous alloys are highly disordered in structure and are metastable. When heated at a relatively high temperature, crystallization reactions will occur, leading to changes in the alloy properties [ 141. According to the Ni-P binary phase diagram [ 151, the eutectic composition is 10 wt.% P and the two phases composing the eutectic structure are nickel (solid solution) and NisP. Therefore, of the alloys used in this investigation, one is eutectic, two are hypo-eutectic and two are hypereutectic. In Figs. 2 - 5, X-ray diffraction patterns for four experimental alloys after annealing for 1 h at different temperatures are shown. It can be seen that for hypo-eutectic alloys, nickel crystals precipitate first from the amorphous parent phase and the simultaneous crystallization of nickel and NisP then follows. The reaction is completed at about 673 K. In hypereutectic alloys, Ni$ precipitates first instead of nickel, while in the eutectic alloy only the simultaneous precipitation of nickel and NiaP is observed. The crystallization process is complete at about 723 K for both the eutectic and the hypereutectic alloys. From the crystallization process cited above, it seems that the amorphous alloys crystallize in a manner similar to the solidification of the corresponding liquid alloys. However, since the crystallization of the amorphous alloys is developed under extremely large supercoolings, the driving force for the transformation is large and the diffusivity of the atoms
272
973K
973K 873K
773K
773K 723K 673K
* .z z 9 2
723K 673K 623K
623K
573K
573K 523K
w
35
473K 35
40 29,
degrees
40 45 ~6, degrees
Fig. 2. X-ray diffraction patterns of the Ni -7.67%P peratures: X, nickel; A, NisP.
alloy after annealing at various tem-
Fig. 3. X-ray diffraction patterns of the Ni -9.61%P peratures: X, nickel; a, Ni3P.
alloy after annealing at various tem-
973K
?.
723I’
.Y
20
673x
2
35
40 26,
45
50
55
-5 35 26,
degrees
Fig. 4. X-ray diffraction patterns peratures: X, nickel; A, Ni3P.
of the Ni-10.38%P
Fig. 5. X-ray diffraction patterns of the Ni-13.56%P peratures: X, nickel; A, NiaP.
degrees
alloy after annealing at various
tem-
alloy after annealing at various tem-
273
is small, it is reasonable that the morphology of the microstructures obtained is quite different from that of the solidification structures [16]. If further annealing is applied after the completion of the crystallization process, the nickel phase will undergo an Ostwald ripening process, thus causing the structure of all the experimental alloys to consist of finely dispersed granular nickel particles embedded in an Ni,P matrix, as shown in Fig. 6.
(4
(b)
(cl Fig. 6. Microstructures of the Ni-lS.l%P alloy after annealing at various temperatures: (a) 873 K; (b) 973 K; (c) 973 K (carbon replica). The specimens were etched electrolytically in 0.5% chromic acid. (Magnifications: (a) 800x;(h) 800X; (c) 13 000x.)
3.3. Effect of 1 h annealing temperature on hardness Figure 7 shows the dependence of the hardness on the 1 h annealing temperature of the experimental alloys. The hardness changes very little with increasing temperature when the annealing temperature is below 523 K, but above that it increases abruptly. All the alloys attain maximum hardness values after heating at a particular temperature; the Ni-13.56%P alloy at about 723 K and the others at about 673 K. As the annealing temperature is further raised, the hardness values fall gradually. From the aforementioned structural changes during crystallization, it can be seen that the
274
800 -
473
673
Annealing Temp.,
873
K
Fig. 7. The dependence of the hardness on the annealing temperature for the five experimental alloys: 0, Ni-7.67%P; *, Ni-9.61%P; 0, Ni-10.38%P; 0, NC12.1%P; X, Ni13.56%P.
marked increase in hardness is due to the formation of the hard intermediate phase Ni& the hardness peaks roughly correspond to the completion of crystallization. The higher the phosphorus content, the greater the NiQ phase content and consequently the higher the peak hardness. The decrease in hardness at higher annealing temperatures can be attributed to the Ostwald ripening process of the nickel particles. 3.4. Wear resistance of electroless deposited Ni-P alloys 3.4.1. Annealing temperature versus wear resistance The relationship between wear volume and annealing temperature is illustrated in Fig. 8. All five alloys show minimum values at a particular annealing temperature; four alloys at about 873 K and the Ni-7.67%P alloy at about 723‘K. Early concepts considered that the wear resistance of metals varies linearly with their hardness. In the present work, however, except for the partially amorphous Ni-7.67%P alloy, the wear resistances of all the other four alloys do not vary in proportion with their hardnesses, i.e. the best wear resistance is not in accord with the greatest hardness. The surface morphology of the wear scar of the crystallized specimens was examined under a scanning electron microscope and the typical appearance is shown in Fig. 9. The wear tracks shown demonstrate that the
275
20 -
0 273
I 673
I 473 Annealing
Temp.,
I a73
1 73
K
Fig. 8. The dependence of the wear volume on the annealing temperature for the five experimental alloys: 0, Ni-7,67%P; A, Ni-961%P; 0, Ni-10.38%P; 0, Ni-lP.l%P; X , Ni-13.56%P.
(4
(b)
Fig. 9. Typical surface morphology of the wear scar of the Ni-9.61%P alloy annealed at 873 K (the wear direction is from left to right): SEM micrographs (secondary electron images). (Magnifications: (a) 55x; (b) 165X.)
nature of the wear involved in the test procedure is fund~en~ly abrasive. The microscopically protruded carbide particles on the hard metal disc can act as abrasive particles. Owing to the extreme brittleness of the crystallized Ni-P alloys, it is thought that some microsp~~ng may occur; which increases during further wear, thus forming the large deep tracks (white area).
276
Recent researchers have pointed out that the wear resistance of metals is dependent on the hardness, the fracture toughness and the microstructure [17,18]. This is reasonable because in general, whenever a wear debris particle is detached from the metal, the processes of elastic deformation, plastic deformation and fracture must occur, with fracture as the final and necessary process. Thus crack propagation in certain microstructures must be taken into account when the formation of wear debris is considered, particularly when the ratio of fracture toughness to hardness of the material is low [ 191. For the alloys used in this investigation, even with the alloy of lowest phosphorus content (7.67%), there would be roughly 50% of the NisP phase in the crystallized state as estimated from the phase diagram by the lever rule. For the other four alloys with higher phosphorus contents there must be an even greater NisP phase content in the crystallized structure, e.g. the estimated NisP content for the 12.1% P alloy is roughly 80%. Therefore the ductile nickel phase must be in the form of isolated particles surrounded by a brittle NisP matrix. Evidently, this sort of structure must be hard and brittle, and the toughness-to-hardness ratio is low. In fact, when the deposited layer is peeled off from the substrate and then crystallized, it is so brittle that it is hard to maintain its completeness without cracking. During the wear process, when a crack in the Ni$ matrix propagates into a nickel particle, the ductile nickel phase will undergo plastic deformation which will blunt the crack tip, relax the stress concentration at the front of the crack and slow down the propagation. If the annealing temperature is low, crystallization cannot be completed and the structure of the alloy is partially amorphous. Because the amorphous phase is tougher, the wear resistance is substantially controlled by the hardness and increases as the hardness increases. After crystallization, the nickel particles agglomerate and become larger. When a crack propagates into such a particle, enough plastic deformation can occur to slow down the growth of the crack and the wear volume can be decreased. If the particle is small, as is the case when crystallization has just finished or when agglomeration of the nickel particles is not far from starting, plastic deformation is restricted to a limited volume and stress concentration can only be relaxed to a limited degree before the crack penetrates through the particle. If the annealing temperature is still higher, the size of the nickel particles further increases, and the average distance between adjacent particles is also increased, so the mean free path for crack propagation becomes larger. This means that when a crack propagates into the matrix, the probability that it encounters a nickel particle is smaller, and so it follows that the wear volume increases and goes through a minimum. 3.4.2. Effect of phosphorus content on wear resistance The data obtained were replotted for the dependence of the wear volume on the phosphorus content, and are shown in Fig. 10 for the asdeposited and annealed (673 and 873 K) conditions. In the as-deposited
277 100
80
0 6
8
10
12
14
P, wt.% Fig. 10. The dependence of the wear volume on the phosphorus content of the five experimental alloys: o, as plated; A, annealed at 673 K; l, annealed at 873 K.
condition all the alloys have similar hardnesses, but the wear volumes decrease monotonically with increasing phosphorus content. When the annealing temperature is 673 K, most of the alloys reach their maximum hardness and the wear volumes are nearly equal. If the annealing temperature is 873 K, the alloys reach a state of best wear resistance, and the three alloys with phosphorus contents of 9.61%, 10.38% and 12.1% produce the least wear volumes. This is possibly due to the fact that at higher phosphorus contents the concentration of ductile nickel particles is so small that they are only rarely encountered by the propagating crack during the wear process. To summarize, it seems that there exists an optimum structure for best wear resistance in the Ni-P alloys, i.e. an optimum amount and size of granular nickel particles dispersed in an NisP matrix. 4. Conclusions (1) After crystallization, all amorphous Ni-P alloys have a structure of granular nickel particles embedded in an Ni$ matrix, although their transformation characteristics differ during the heat treatment. The higher the phosphorus content, the greater the Ni$ phase content. (2) The unusual increase in hardness after heat treatment is the result of crystallization. If the alloy is completely amorphous in the as-deposited
condition, the maximum hardness is reached after annealing at 673 - 723 K for 1 h. (3) Alloys with phosphorus contents of 9% - 13% which are annealed at about 873 K exhibit the best wear resistance, but this does not coincide with the highest hardness; this can be related to the structure of the alloys.
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