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Dynamic abrasion resistance of coatings applied to engineering materials
Surface and Coatings Technology, 68/69 (1994) 477—481
477
Dynamic abrasion resistance of coatings applied to engineering materials D.M. Kennedya, M. Helalib, M.S.J. Hashmib ~School of Mechanical Engineering, RTC Carlow, Carlow, Ireland bschool of Mechanical and Manufacturing Engineering, Dublin City University, Dublin, Ireland
Abstract Combined impact and abrasion tests were conducted on coated and uncoated samples of aluminium, mild steel, and tool steels using a purpose built test rig. Tungsten carbide—cobalt (WC—Co) coatings ofdifferent thicknesses were deposited using a high velocity oxyfuel (HVOF) process. Experimental work showed that wear rates were greater under impact abrasion conditions compared with pure contact abrasion. As the substrate hardness increased, the wear rates decreased under the test conditions. Coated aluminium samples experienced greater wear under impact conditions than uncoated samples. This was due to deformation of the substrate which led to cracking and spalling of the coatings. Impact abrasion produced greater wear on the mild steel than pure abrasion. The tool steels experienced very low wear rates under both test conditions and only slight polishing of the surfaces occurred. Results show that the wear rate on each sample was not affected by the coating thickness, but the coating thickness did influence the wear resistance.
1. Introduction The main aim of this research was to compare the effects of combined impact and abrasion with contact abrasion on coatings of different thicknesses, applied to a number of substrate materials. Uncoated samples were subjected to identical testing and their wear resistance was compared with that of the coated samples. Operations involving wear conditions of combined impact and abrasion include punching, milling, peat production and forging. In the majority of metalworking and metal to metal contact processes, materials are deformed by stresses normal to the surfaces. The relative movement of surfaces generates shear stresses at the interface, giving rise to tribological conditions. Tribology is the study of adhesion, friction, lubrication and wear of materials [1,2]. Impact refers to the collision of two masses with initial relative velocity [3,4]. Materials and alloys that are resistant to repeated impacts generally are not very resistant to abrasion and vice versa [5,6]. Abrasive wear occurs when a soft surface with hard particles slides along a hard surface, ploughing a series of grooves in that surface [7,8]. Resistance to contact abrasion and impact is a prime requirement for many materials applications, and tungsten carbide—cobalt (WC—Co) is applied mainly for the prevention of wear [9]. Vinogradov et al. [10] and others [11,12] showed that the hardness or its equivalent cannot characterize uniquely the magnitude of wear under the conditions of abrasive impact. Abrasion studies have shown that the microstructure
0257—8972/94/$7.OO SSDI 0257-8972(94)08044-Y
determines the abrasive wear resistance of the coating. For example, the wear decreases with the carbide size. Small carbides do not fracture easily and if a crack develops it has difficulty progressing. Finer grains of WC can interlock mechanically and resist removal. Once individual grains are removed, enhanced removal of adjacent grains occurs. Larger tungsten carbide grains have a lower fracture strength and are thus expected to crack more easily.
2. High velocity oxy-fuel (HVOF) process The HVOF spray process uses an oxygen—propylene—air gas mixture. The HVOF process used for applying the coatings in this research work and shown in Fig. 1 consists of a manually operated diamond jet gun, a powder feed unit, a flow rate meter, air supply and control unit, a spray booth and extractor system. The coating powder is carried by nitrogen and fed into the centre of the flame. It is heated to its molten or semimolten state and is accelerated towards the substrate surface [13]. Owing to the high velocity and high impact, the coating is less porous than that produced by normal thermal spray processes, and has good bonding strength to the substrate.
3. Experimental procedure The surface roughnesses of the substrates and coatings plus the coating thicknesses were measured and are
© 1994
—
Elsevier Science S.A. All rights reserved
478
D.M. Kennedy et al.
/
Dynamic abrasion resistance POWDER FEED UNIT
AIR COMPRESSOR
AIR CONTROL UNIT
_~7
FLOW METER UINT
___
SOUND PROOF ROOM
=
PROPYLENE FUEL TANKS
GUN
________
EXTRACTOR~___
NITROGEN
—
DEN TANKS
Fig. 1. High velocity oxy-fuel (HVOF) system.
shown in Table 1. Wear tests were carried out using normal and impact forces at sliding velocities ranging between 0 and 0.11 m s~. Samples of aluminium, mild steel and tool steels (En 24 and AISI H13) were used as the substrate materials. All substrates were grit blasted and coatings were applied shortly following this process. Diamalloy 2003, tungsten carbide—cobalt (WC—Co) powder, was used in the process. Coatings were applied to each of the substrates and these were classified into four main thicknesses. The powder size was less than 40 p.m and coarse in appearance. Table 1 Surface roughness, hardness, (H~
3~ for samples Al and MS, R~for samples En24 and H13) and coating thickness Sample
Surface roughness Ra
Coating thickness (mm)
substrate (jim)
Surface roughness coating (tim)
5.5
0.517
4.5
Al-3 AI-4 MS-i MS-2 MS-3
55
0232
01043
415
0.384 0.460 0.171
4.5 4.5 4.5
En24-2 En24-3 En24-4 H13-i H13-2 Hi3-3 H13-4
7.0 7.0 6.5 3.2 3.2 3.2 3.5
6.2 6.2 6.2
Substrate
Coating
5. Test results 70
1089 I
90 318 318 318
974
1370 1272 1400
~ 0.450 0.170 0.044 0.386 0.455 0.200 0.050
All tests were performed on the impact abrasion test rig shown in Fig. 2 for a specified number of cycles. In operation, a round nose stylus of 2.5 mm radius impacts the test specimen in the z-axis and then abrades along it in the x-axis as shown in Fig. 3. The stylus indenter was produced from ISO P50 carbide grade. The contact abrasion distance was 20 mm, and the normal force applied was 100 N. The test rig can perform direct impact abrasion, pure contact abrasion, and combined impact and abrasion. All tests were carried out under unlubricated conditions. Further details of this test rig can be found in the literature [14]. An on-line data acquisition system combined with a force transducer provided measurements of the normal force acting on the stylus in real time during testing.
Ra
Al-i
515
Microhardness H~300
4. Test rig
7.0 7.0 7.0 5.0 4.7 4.8 5.0
Al aluminium, MS mild steel; Hi3 is AISI H13.
52 52 52 50.5 50.5 50.5 50.5
1321 1251 1255 1245 1250 1268 1328
The mass loss of the coating/substrate samples was noted at regular intervals during the test process and graphical results show material loss vs. number of cycles for both impact abrasion and contact abrasion. The depth of the crater and the wear track produced during the tests was measured and comparisons were made for the coatings and substrates used. Figs. 4—7 show the wear rates due to impact and abrasion on the coating/ substrate combinations of aluminium and mild steel. Figs. 8 and 9 show the depth of the wear track and crater due to abrasion and impact respectively. The amount of material removed was low for the four main .
substrates used in the contact abrasion tests and observation showed small particles being removed under these test conditions. Owing to combined impact and abrasion
D.M. Kennedy et al.
/
Dynamic abrasion resistance
479
CONTROL. UNIT.
Loo.cl
C
1
1.1.
—
COMPUTER
IMPACT TOOL
FIXED BASE PLATE
~
IPRINTER
I
2,500
3,000
j
______
Fig. 2. Impact abrasion test rig.
~
~Hardened
::
Plate
~
Fig. 3. Stylus motion under impact and abrasion conditions.
the coatings on the softer substrates experienced considerable wear. This was due to cracking, cratering, abrasion and plastic deformation. Under combined impact abrasion conditions the wear rates increased, especially for the softer substrate samples with large particle size. It is shown in Fig. 6 that the WC—Co coatings on aluminium samples experienced greater wear under impact conditions than the uncoated samples. This effect was not found for the same test conditions. Impact abrasion produced greater wear on the mild steel than contact abrasion. Graphical results show that thin coatings performed best on soft aluminium substrates subjected to impact abrasion conditions whereas the thicker coating performed better on the mild steel sample for the same test.
0.OE+00
500
1,000
1,500
2,000
3,500
CYCLES Fig. 4. Impact abrasion tests on coated and uncoated mild steel: * uncoated sample, x MS-i, ~ MS-2, + MS-3, I MS-4.
6. Conclusions From the results obtained it is shown that the characteristics of the substrate material have a major effect on the wear resistance of the coatings examined under impact conditions. The coatings employed improved the wear resistance of the samples under contact abrasion conditions. Coatings that performed satisfactorily under contact abrasion showed high wear rates under impact abrasion conditions. Hard WC—Co coatings applied to soft substrates such
480
D.M. Kennedy et al.
/
Dynamic abrasion resistance
8.0E-O3~
35
F
30 6.OE-03~25 .3 ~20 a-
~ 4.OE-03-
I
I
FI
w
w
I
F
I
~15
~ioI~.
2.OE-03 -
0.OE+00 * 500
,
1,000
1,500
~ 2,000
1 ____
o~-~ 2,500
3,000
II
I
w
3,500
1
2
~
____ 3
4
CYCLES
5
6
7
SAMPLES
Fig. 5. Direct abrasion tests on coated and uncoated mild steel: * uncoated sample, x MS-i, ~ MS-2, + MS-3, I MS-4.
Fig. 8. Wear track depth for aluminium and mild steel samples under abrasion conditions: 1 MS uncoated, 2 MS-i, 3 aluminium uncoated, 4 Al-i, 5 Al-2, 6 Ai-3, 7 Al-4.
1 .2E-02 700 1 .0E~02 600 E 500
8.OE-03
I
~
w
6.OE-03
~ 400
w ID
rE300 w
4.OE-03 2.OE-03 0.OE+00 500
//77
200 .
1,000
1,500
2,000
2,500
3,000
3,500 1
CYCLES Fig. 6. Direct abrasion tests on coated and uncoated aluminium: + uncoated sample, * Al-i, • Al-2, x Al-3, ~ Al-4.
2
3
4
5
6
7
SAMPLES Fig. 9. Crater depth for aluminium and mild steel samples under impact conditions: 1 MS uncoated, 2 MS-i, 3 aluminium uncoated, 4 Al-i, 5 Al-2, 6 Al-3, 7 Al-4.
1 .2E-01 1 .OE-01
of the material was high owing to the effects of impact forces. The coated and uncoated tool steels used in the testing showed very low levels of wear compared with the other substrate materials used, and extended tests would be
8.OE-02 ~
6.OE-02
w 0.OE+00 4.OE-02 2.OE-02 500
1,000
1,500
2,000
2,500
3,000
3,500
required as aluminium to assess weretheir destroyed wear properties rapidly andsatisfactorily. the wear rates References [1] R. Chattopadhyay, Advances in Wear Protection Methods and Systems, Proc. 9th Int. Conf. on Industrial Tribology, India, 1991,
CYCLES
Fig. 7. Impact abrasion tests on coated and uncoated aluminium: + uncoated sample, * Al-i, • Al-2, LI A1-3, x Al-4.
pp. miiS—m123. [2] K.H. ZumGahr, Microstructure and wear of materials, Elsevier Tribology Series Vol. 10, Elsevier, Amsterdam, 1987, p. V. [3] MC. Vagle, Modern Machine Shop, (March 1990) 84.
D.M. Kennedy et al.
/
[4] S.S. Brenner, HA. Wriedt and R.A. Oriani, Wear, 68 (1981) 169—190. [5] J. FohI, T. Weissenberg and J. Wiedemeyer, Wear, 130 (1) (1989) 275—288. [6] R. Blickensderfer and J.H. Tylczak, Report of Investigation, Information Circular 9001, 1985 (Department of the Interior, Bureau of Mines, Washington, DC). [7] B.J. Gill, Manufact. Eng., (October 1985) 23. [8] 5. Grainger (ed), Engineering Coatings, Design and Application, Abington Publishing, 1989, p. 189. [9] H. Uetz and C. Hanser, Abrasion and Erosion, Carl Hanser Verlag, 1986.
Dynamic abrasion resistance
481
[10] V.N. Vinogradov, G.M. Sorokin and A.Yu. Albagachiev, Mashinostroenie, Mashinostroenie, Moscow, 1982. [ii] G.M. Sorokin, Mashinovedenie, 3 (1973) 111—115. [12] Yu.V. Kolesnikov, Vestn. Mashinostr., 70(6) (1990) 16—19. [13] M. Helali and M.S.J. Hashmi, in M.S.J. Hashmi (ed), Advances in Materials and Processing Technology, Vol. 2, Dublin City University Press, 1993, pp. 13 15—1322. [14] D. Kennedy and M.S.J. Hashmi, in M.S.J. Hashmi(ed.), Advances in Materials and Processing Technology, Vol. 3, Dublin University Press, 1993, pp.2057—2069.
477
Dynamic abrasion resistance of coatings applied to engineering materials D.M. Kennedya, M. Helalib, M.S.J. Hashmib ~School of Mechanical Engineering, RTC Carlow, Carlow, Ireland bschool of Mechanical and Manufacturing Engineering, Dublin City University, Dublin, Ireland
Abstract Combined impact and abrasion tests were conducted on coated and uncoated samples of aluminium, mild steel, and tool steels using a purpose built test rig. Tungsten carbide—cobalt (WC—Co) coatings ofdifferent thicknesses were deposited using a high velocity oxyfuel (HVOF) process. Experimental work showed that wear rates were greater under impact abrasion conditions compared with pure contact abrasion. As the substrate hardness increased, the wear rates decreased under the test conditions. Coated aluminium samples experienced greater wear under impact conditions than uncoated samples. This was due to deformation of the substrate which led to cracking and spalling of the coatings. Impact abrasion produced greater wear on the mild steel than pure abrasion. The tool steels experienced very low wear rates under both test conditions and only slight polishing of the surfaces occurred. Results show that the wear rate on each sample was not affected by the coating thickness, but the coating thickness did influence the wear resistance.
1. Introduction The main aim of this research was to compare the effects of combined impact and abrasion with contact abrasion on coatings of different thicknesses, applied to a number of substrate materials. Uncoated samples were subjected to identical testing and their wear resistance was compared with that of the coated samples. Operations involving wear conditions of combined impact and abrasion include punching, milling, peat production and forging. In the majority of metalworking and metal to metal contact processes, materials are deformed by stresses normal to the surfaces. The relative movement of surfaces generates shear stresses at the interface, giving rise to tribological conditions. Tribology is the study of adhesion, friction, lubrication and wear of materials [1,2]. Impact refers to the collision of two masses with initial relative velocity [3,4]. Materials and alloys that are resistant to repeated impacts generally are not very resistant to abrasion and vice versa [5,6]. Abrasive wear occurs when a soft surface with hard particles slides along a hard surface, ploughing a series of grooves in that surface [7,8]. Resistance to contact abrasion and impact is a prime requirement for many materials applications, and tungsten carbide—cobalt (WC—Co) is applied mainly for the prevention of wear [9]. Vinogradov et al. [10] and others [11,12] showed that the hardness or its equivalent cannot characterize uniquely the magnitude of wear under the conditions of abrasive impact. Abrasion studies have shown that the microstructure
0257—8972/94/$7.OO SSDI 0257-8972(94)08044-Y
determines the abrasive wear resistance of the coating. For example, the wear decreases with the carbide size. Small carbides do not fracture easily and if a crack develops it has difficulty progressing. Finer grains of WC can interlock mechanically and resist removal. Once individual grains are removed, enhanced removal of adjacent grains occurs. Larger tungsten carbide grains have a lower fracture strength and are thus expected to crack more easily.
2. High velocity oxy-fuel (HVOF) process The HVOF spray process uses an oxygen—propylene—air gas mixture. The HVOF process used for applying the coatings in this research work and shown in Fig. 1 consists of a manually operated diamond jet gun, a powder feed unit, a flow rate meter, air supply and control unit, a spray booth and extractor system. The coating powder is carried by nitrogen and fed into the centre of the flame. It is heated to its molten or semimolten state and is accelerated towards the substrate surface [13]. Owing to the high velocity and high impact, the coating is less porous than that produced by normal thermal spray processes, and has good bonding strength to the substrate.
3. Experimental procedure The surface roughnesses of the substrates and coatings plus the coating thicknesses were measured and are
© 1994
—
Elsevier Science S.A. All rights reserved
478
D.M. Kennedy et al.
/
Dynamic abrasion resistance POWDER FEED UNIT
AIR COMPRESSOR
AIR CONTROL UNIT
_~7
FLOW METER UINT
___
SOUND PROOF ROOM
=
PROPYLENE FUEL TANKS
GUN
________
EXTRACTOR~___
NITROGEN
—
DEN TANKS
Fig. 1. High velocity oxy-fuel (HVOF) system.
shown in Table 1. Wear tests were carried out using normal and impact forces at sliding velocities ranging between 0 and 0.11 m s~. Samples of aluminium, mild steel and tool steels (En 24 and AISI H13) were used as the substrate materials. All substrates were grit blasted and coatings were applied shortly following this process. Diamalloy 2003, tungsten carbide—cobalt (WC—Co) powder, was used in the process. Coatings were applied to each of the substrates and these were classified into four main thicknesses. The powder size was less than 40 p.m and coarse in appearance. Table 1 Surface roughness, hardness, (H~
3~ for samples Al and MS, R~for samples En24 and H13) and coating thickness Sample
Surface roughness Ra
Coating thickness (mm)
substrate (jim)
Surface roughness coating (tim)
5.5
0.517
4.5
Al-3 AI-4 MS-i MS-2 MS-3
55
0232
01043
415
0.384 0.460 0.171
4.5 4.5 4.5
En24-2 En24-3 En24-4 H13-i H13-2 Hi3-3 H13-4
7.0 7.0 6.5 3.2 3.2 3.2 3.5
6.2 6.2 6.2
Substrate
Coating
5. Test results 70
1089 I
90 318 318 318
974
1370 1272 1400
~ 0.450 0.170 0.044 0.386 0.455 0.200 0.050
All tests were performed on the impact abrasion test rig shown in Fig. 2 for a specified number of cycles. In operation, a round nose stylus of 2.5 mm radius impacts the test specimen in the z-axis and then abrades along it in the x-axis as shown in Fig. 3. The stylus indenter was produced from ISO P50 carbide grade. The contact abrasion distance was 20 mm, and the normal force applied was 100 N. The test rig can perform direct impact abrasion, pure contact abrasion, and combined impact and abrasion. All tests were carried out under unlubricated conditions. Further details of this test rig can be found in the literature [14]. An on-line data acquisition system combined with a force transducer provided measurements of the normal force acting on the stylus in real time during testing.
Ra
Al-i
515
Microhardness H~300
4. Test rig
7.0 7.0 7.0 5.0 4.7 4.8 5.0
Al aluminium, MS mild steel; Hi3 is AISI H13.
52 52 52 50.5 50.5 50.5 50.5
1321 1251 1255 1245 1250 1268 1328
The mass loss of the coating/substrate samples was noted at regular intervals during the test process and graphical results show material loss vs. number of cycles for both impact abrasion and contact abrasion. The depth of the crater and the wear track produced during the tests was measured and comparisons were made for the coatings and substrates used. Figs. 4—7 show the wear rates due to impact and abrasion on the coating/ substrate combinations of aluminium and mild steel. Figs. 8 and 9 show the depth of the wear track and crater due to abrasion and impact respectively. The amount of material removed was low for the four main .
substrates used in the contact abrasion tests and observation showed small particles being removed under these test conditions. Owing to combined impact and abrasion
D.M. Kennedy et al.
/
Dynamic abrasion resistance
479
CONTROL. UNIT.
Loo.cl
C
1
1.1.
—
COMPUTER
IMPACT TOOL
FIXED BASE PLATE
~
IPRINTER
I
2,500
3,000
j
______
Fig. 2. Impact abrasion test rig.
~
~Hardened
::
Plate
~
Fig. 3. Stylus motion under impact and abrasion conditions.
the coatings on the softer substrates experienced considerable wear. This was due to cracking, cratering, abrasion and plastic deformation. Under combined impact abrasion conditions the wear rates increased, especially for the softer substrate samples with large particle size. It is shown in Fig. 6 that the WC—Co coatings on aluminium samples experienced greater wear under impact conditions than the uncoated samples. This effect was not found for the same test conditions. Impact abrasion produced greater wear on the mild steel than contact abrasion. Graphical results show that thin coatings performed best on soft aluminium substrates subjected to impact abrasion conditions whereas the thicker coating performed better on the mild steel sample for the same test.
0.OE+00
500
1,000
1,500
2,000
3,500
CYCLES Fig. 4. Impact abrasion tests on coated and uncoated mild steel: * uncoated sample, x MS-i, ~ MS-2, + MS-3, I MS-4.
6. Conclusions From the results obtained it is shown that the characteristics of the substrate material have a major effect on the wear resistance of the coatings examined under impact conditions. The coatings employed improved the wear resistance of the samples under contact abrasion conditions. Coatings that performed satisfactorily under contact abrasion showed high wear rates under impact abrasion conditions. Hard WC—Co coatings applied to soft substrates such
480
D.M. Kennedy et al.
/
Dynamic abrasion resistance
8.0E-O3~
35
F
30 6.OE-03~25 .3 ~20 a-
~ 4.OE-03-
I
I
FI
w
w
I
F
I
~15
~ioI~.
2.OE-03 -
0.OE+00 * 500
,
1,000
1,500
~ 2,000
1 ____
o~-~ 2,500
3,000
II
I
w
3,500
1
2
~
____ 3
4
CYCLES
5
6
7
SAMPLES
Fig. 5. Direct abrasion tests on coated and uncoated mild steel: * uncoated sample, x MS-i, ~ MS-2, + MS-3, I MS-4.
Fig. 8. Wear track depth for aluminium and mild steel samples under abrasion conditions: 1 MS uncoated, 2 MS-i, 3 aluminium uncoated, 4 Al-i, 5 Al-2, 6 Ai-3, 7 Al-4.
1 .2E-02 700 1 .0E~02 600 E 500
8.OE-03
I
~
w
6.OE-03
~ 400
w ID
rE300 w
4.OE-03 2.OE-03 0.OE+00 500
//77
200 .
1,000
1,500
2,000
2,500
3,000
3,500 1
CYCLES Fig. 6. Direct abrasion tests on coated and uncoated aluminium: + uncoated sample, * Al-i, • Al-2, x Al-3, ~ Al-4.
2
3
4
5
6
7
SAMPLES Fig. 9. Crater depth for aluminium and mild steel samples under impact conditions: 1 MS uncoated, 2 MS-i, 3 aluminium uncoated, 4 Al-i, 5 Al-2, 6 Al-3, 7 Al-4.
1 .2E-01 1 .OE-01
of the material was high owing to the effects of impact forces. The coated and uncoated tool steels used in the testing showed very low levels of wear compared with the other substrate materials used, and extended tests would be
8.OE-02 ~
6.OE-02
w 0.OE+00 4.OE-02 2.OE-02 500
1,000
1,500
2,000
2,500
3,000
3,500
required as aluminium to assess weretheir destroyed wear properties rapidly andsatisfactorily. the wear rates References [1] R. Chattopadhyay, Advances in Wear Protection Methods and Systems, Proc. 9th Int. Conf. on Industrial Tribology, India, 1991,
CYCLES
Fig. 7. Impact abrasion tests on coated and uncoated aluminium: + uncoated sample, * Al-i, • Al-2, LI A1-3, x Al-4.
pp. miiS—m123. [2] K.H. ZumGahr, Microstructure and wear of materials, Elsevier Tribology Series Vol. 10, Elsevier, Amsterdam, 1987, p. V. [3] MC. Vagle, Modern Machine Shop, (March 1990) 84.
D.M. Kennedy et al.
/
[4] S.S. Brenner, HA. Wriedt and R.A. Oriani, Wear, 68 (1981) 169—190. [5] J. FohI, T. Weissenberg and J. Wiedemeyer, Wear, 130 (1) (1989) 275—288. [6] R. Blickensderfer and J.H. Tylczak, Report of Investigation, Information Circular 9001, 1985 (Department of the Interior, Bureau of Mines, Washington, DC). [7] B.J. Gill, Manufact. Eng., (October 1985) 23. [8] 5. Grainger (ed), Engineering Coatings, Design and Application, Abington Publishing, 1989, p. 189. [9] H. Uetz and C. Hanser, Abrasion and Erosion, Carl Hanser Verlag, 1986.
Dynamic abrasion resistance
481
[10] V.N. Vinogradov, G.M. Sorokin and A.Yu. Albagachiev, Mashinostroenie, Mashinostroenie, Moscow, 1982. [ii] G.M. Sorokin, Mashinovedenie, 3 (1973) 111—115. [12] Yu.V. Kolesnikov, Vestn. Mashinostr., 70(6) (1990) 16—19. [13] M. Helali and M.S.J. Hashmi, in M.S.J. Hashmi (ed), Advances in Materials and Processing Technology, Vol. 2, Dublin City University Press, 1993, pp. 13 15—1322. [14] D. Kennedy and M.S.J. Hashmi, in M.S.J. Hashmi(ed.), Advances in Materials and Processing Technology, Vol. 3, Dublin University Press, 1993, pp.2057—2069.