- Email: [email protected]
Abrasive wear behavior of high speed steel and hard metal coated with TiAlN and TiCN
Wear 250 (2001) 561–568
Abrasive wear behavior of high speed steel and hard metal coated with TiAlN and TiCN J.D. Bressan a,∗ , R. Hesse a , E.M. Silva Jr. b a
Departmento de Engenharia Mecânica, Centro de Ciências Tecnológicas — CCT/UDESC Campus Universitário, 89223-100 Joinville/SC, Brazil b Fabrica de Componentes, B1. 14 EMBRACO, Rua Rui Barbosa 1020, Cx. P. 91 89.219-901 Joinville/SC, Brazil
Abstract The wear behavior of M2 high speed HSS steel and WC hard metal coated with TiAlN and TiCN were investigated and compared, using the pin on disk standard test with different loads. The coating PVD process has been done by two different suppliers, using an industrial equipment unit with optimized conditions. The coated layers were measured and characterized. The load, sliding distance and velocity of 0.5 m/s were kept constant during the abrasion test in order to control these variables. The counterface disks used were electric steel sheets from three different suppliers. The lost volume and temperature at the pin end have been measured during the wear test. Comparisons of tribological performance for the coated HSS and hard metal were done, using a plot of lost volume versus sliding distance for substrates and coatings. The pin worn surfaces were observed using a scanning electron microscope. A significant increase in the wear resistance of M2 steel and WC hard metal when coated with TiAlN and TiCN was observed. Quality of these coatings depended upon the supplier. Excessive porosity has diminished the TiAlN counting wear resistance from one supplier. However, in general the performance of TiAlN is superior to TiCN. The pin wear rate depended on the disk microstructure. © 2001 Elsevier Science B.V. All rights reserved. Keywords: High speed steel; Hard metal; TiAlN; TiCN; Wear
1. Introduction The classes of processes for surface modification or coating aimed at improving performance by obtaining a good combination of surface and bulk properties not attainable by other means, is known as surface engineering [1]. The two main objectives of surface engineering for tribological applications in components and tooling are: increase wear resistance and modify friction behavior. In some cases both aims are attained. Recently, the industrial and medical applications of wear resistant materials and coatings as well as their economic implications have received much attention from researchers and institutes. It has been of great concern to advance the properties of existing materials and to develop new materials capable of giving better in-service performance of components to the designed functions. Many new wear resistant materials have been presented for quite different applications, but increase in wear resistance has been attained with great efficiency by various coatings and surface treatment techniques.
∗ Corresponding author. E-mail addresses: [email protected] (J.D. Bressan), eraclito m silva [email protected] (E.M. Silva Jr.).
Wear of machinery parts and tooling has direct influence on productivity, efficiency, reliability and quality of manufactured equipment and products. Therefore, it has been an issue of great concern due to tool life, components repair, failure and undesirable production line stops. Besides, future tendency is to increase the speed of production and use of lighter materials. Consequently, velocity and stress levels of machinery and tools will increase and so the wear problems will increase also. Wear is a complex surface phenomenon that occurs mainly due to sliding and impact of hard particles against the solid surfaces, and corrosion, even in the presence of lubricants. It plays an important role in the life of tools and machine components due to the loss of mass of the material, fatigue and failure problems arising from the increasing surface roughness and cracks. Friction arises from the interaction of microscopic asperities or roughness found in all solid surfaces [2]. The surface interactions are mechanical deformation and chemical adhesion of asperities leading to high forces necessary to promote sliding. The effective contact area between two solids is usually a small part of the nominal area and the contact pressure at the level of the roughness, in general, can be much greater than the elastic limit of the material which leads to plastic deformation.
0043-1648/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 6 3 8 - X
562
J.D. Bressan et al. / Wear 250 (2001) 561–568
Wear mechanisms that cause material removal from the surface include: ploughing, wedge formation, micro-chip formation, chemical adhesion, erosion, corrosion, fretting, gouging, scuffing, galling and delamination [1–4]. However, the abrasive wear mechanisms involving plastic deformation mechanisms [4–8] are: ploughing, wedge formation and micro-cutting. In the ploughing mode, a plastic wave is pushed continuously in front of the hard particle and no material is removed. In the wedge formation mechanism, there is the development of a wedge or built-up-edge in front of the sliding hard particle that eventually is sheared off and another wedge is formed. On the other hand, in the cutting mode, formation of a continuous chip in front of the sliding asperity or particle occurs. Both wedge formation and cutting promotes material removal. Numerical methods have also been used to simulate friction and wear [9]. Despite these recent developments in friction and wear theories, which can predict reasonably well the main wear mechanism, and advanced manufacturing techniques, there are still component wear losses due to the lack of design optimization and the presence of contaminant particles from the environment. In abrasion there are the inevitable asperities contacts and these hard particles contaminants. Thus, to reduce wear, it is necessary to increase the surface hardness of components and tooling to values greater than, or close to, the hardness of these hard particles. However, for some applications, wear resistant bulk material may not be appropriate due to cost, weight, other mechanical properties or manufacturing difficulties. For such situations, surface engineering methods can be employed either to apply a hard coating on a substrate with good bulk properties, or to modify the surface properties by changing the surface material phase or chemical composition. Surface engineering processes for tribological applications in components and tooling are therefore necessary in order to increase the material wear resistance or to modify, its friction behavior. The surface and bulk of a material component often need to satisfy different requirements. Better overall performance can be obtained if the surface struc-
ture and properties can be modified, without damaging the underlying bulk material or substrate. The available surface treatment processes should be considered as an integral part of the design and material selection. Costs may be reduced by selecting a cheaper alloy for bulk properties and enhancing only the surface. The surface engineering methods can be classified in two broad classes [1]: surface modification and surface coating. In the first case, the material surface microstructure can be modified selectively without changing its composition as in the quenching process or in the laser surface melting, followed by fast solidification process. Alternatively, composition and microstructure can be modified simultaneously. This can be attained by increasing the thermal diffusion of a chemical compound into the surface. The composition and micro-structural changes which can be attained by these methods are obviously limited, and for may purpose film deposition or surface coating can be used. Some techniques are only applicable to metals and some exclusively to steels, although others can be applied to polymers and ceramics. A comparative study of the wear behavior of PVD coatings TiAlN and TiCN were carried out in the present work. The coating process has been done by two different suppliers, using an industrial equipment unit under maximized conditions. The wear performance was experimentally investigated using the pin on disk method. The pins were coated HSS steel and hard metal WC, and the disks were made from electric steel sheets.
2. Materials and experimental procedure PVD coating of the pin surfaces processes was carried out in industrial equipment at a specialized company: coating suppliers A and B. The nominal characteristics of the deposited coatings are shown in Table 1. The hardness of the coatings and the substrate materials have been experimentally measured and the values are seen in Table 2.
Table 1 Nominal characteristics of the deposited TiAlN and TiCN coatings given by suppliers Coating
Microhardness (HV 0.05)
Thickness of coating (m)
Process temperature (◦ C)
Friction coefficient against steel ()
TiCN TiAlN TiAlN × multilayer
3000 3500 3000
1–4 1–3 1–5
≤500 ≤550 ≤550
0.4 0.4 0.4
Table 2 Experimental microhardness results for pin materials substrates and the deposited TiAlN and TiCN coatings from suppliers A and B Material\coating
Microhardness (HV 0.5)
HSS
775
WC
1370
HSS\TiCN
HSS\TiAlN
WC\TiAlN
A
B
A
B
A
B
836
1015
940
–
1730
1675
J.D. Bressan et al. / Wear 250 (2001) 561–568
563
for the hard metal. The pin sliding velocity during the test was kept constant at about 0.5 m/s and the radius of circular wear track was 16 mm. The counterface disks were tested as received and were made of 0.5 mm electric steel sheets received from three different suppliers: steel 1, steel 2 and steel 3 with Vickers hardness 103, 156 and 144 kg/mm2 , respectively. These disks, 62 mm in diameter, were glued to a steel disk base which was held by a chuck jaw and was driven by a small electric motor. The materials were tested by couples and were submitted to similar nominal abrasive conditions. The lost volume of the pin and disk materials were calculated by its measured mass variations. Every 200 m of sliding distance the test was automatically interrupted and the mass variation of pin and disk were
Fig. 1. Pin geometry.
The HSS steel grade M2 pin material was received from Villares of Brazil with chemical composition: 0.85% C; 0.30% Mn; 0.25% Si; 2% V; 4% Cr; 6% W; 5% Mo. The WC hard metal alloy pin material had 12% Co and grain size varying from 1 to 2 m. The electric steel disk materials had chemical composition: 0.10% C; 0.20% Al; 2.1% Si; 0.15% Mn; 0.04% P; 0.008% S. The wear test were run in a pin on disk laboratory equipment with constant load and velocity, according to the ASTM G-99-95 standard, and under normal atmosphere conditions of about 20◦ C and 54–60% relative humidity. The pin contact ends had a curvature radius of 10 mm and were milled to give good surface finish (Fig. 1). The applied normal load were 19.5 and 29.5 N corresponding to a maximum Hertzian contact pressure [5] of approximately 760 and 870 MPa for the HSS steel, and 970 and 1120 MPa
Fig. 2. Mean temperature at the pin end for HSS under normal loads of 19.5 and 29.5 N.
Fig. 3. Microstructure of the electric steel disk surfaces, showing grain size, and inclusion, particles and Vickers hardness.
564
J.D. Bressan et al. / Wear 250 (2001) 561–568
Fig. 4. Comparisons of rate of lost mass for pins under the normal load of 38.6 N, sliding against steel disks 1 and 2.
measured using an analytic balance accurate to 0.1 mg. The lost volume of the pins was calculated by dividing the measured lost mass by the pin material density: 7.85 g/cm3 for HSS steel and 15.7 g/cm3 for WC hard metal. Temperature of the pin contact end was measured during the tests, using a thermocouple inserted at a hole in the pin tip end. The temperature point was 2 mm from the disk contact surface. The maximum temperature attained during the test has been registered and is commented next.
3. Wear behavior and results The experimental results obtained from the pin on disk tests were plotted to show temperature and the lost volume versus the sliding distance for constant velocity and normal load. In all cases there were the formation of scratches on the steel disk after the first 200 m sliding distance. In Fig. 2, the mean temperatures attained during the wear test at the pin end is shown. The point of measurement is
about 2 mm from the contact surface. Temperature rises as the load increases, but are below 70◦ C for HSS and WC. The disk microstructures are different as can be seen in Fig. 3, although they are similar electric steels. The grain size varies quite a lot and hard particles are possibly present in the grain. In Fig. 4, a comparison of the lost mass rate for HSS, HSS coated with TiCN (HSS\TiCN) and WC pins are plotted as functions of the sliding distance. The benefit of the TiCN coating is very clear. The rate of lost mass decreases with the sliding distance possibly due to the decrease in the nominal surface contact pressure as the contact channel in the disk widened. The wear resistance comparison between HSS\TiCN and the hard metal WC, in terms of pin lost mass is misleading as the tool life is related more properly to the lost volume at its edges. The density of the hard metal is double the density of the HSS. Thus, the lost volume for the hard metal is less than the HSS for the same amount of lost mass. In fact, the hard metal lost volume is half the lost volume of the HSS. Therefore, the lost volume is more
Fig. 5. Pin’s lost volume vs. sliding distance for HSS coated with TiCN from suppliers A and B. Normal load of 29.5 N is indicated by II.
J.D. Bressan et al. / Wear 250 (2001) 561–568
565
Fig. 6. (a) and (b) Pin’s lost volume vs. sliding distance. WC coated with TiAlN from suppliers B and WC without coating. Normal load of 19.5 N (I) and 29.5 N (II).
Fig. 7. Pin’s lost volume vs. sliding distance. WC coated with TiAlN from suppliers B and WC coated with TiAlN × multilayer from supplier A. Normal load of 19.5 N (I).
566
J.D. Bressan et al. / Wear 250 (2001) 561–568
Fig. 8. Scanning electron microscope photographs of HSS pin tips coated with TiAlN and TiCN from supplier A, after sliding 2400 m over steel disks 1, 2 and 3. Normal load of 19.5 N.
Fig. 9. Scanning electron microscope photographs of WC pin tips coated with TiAlN multilayers on hard metal pin tips, after sliding 200 m over steel disks 1, 2 and 3. Normal load of 29.5 N. Porosity and hard particles and present at the coating. Abrasive scratches are also observed. Coating supplier A.
J.D. Bressan et al. / Wear 250 (2001) 561–568
567
Fig. 10. Scanning electron microscope photographs of HSS pin tips coated with TiCN after sliding 2000 m over steel disks 1, 2 and 3. Normal load of 19.5 N. Coating process from supplier B.
appropriate to measure the wear resistance of the pin materials and coatings, and is used in the following graphs. In Fig. 5, a comparison of the wear performance of the HSS coated with TiCN from suppliers A and B is made. In general, the performance of the coating from company B is better than the coating delivered by company A, although the nominal specifications are the same. This is possibly due to variations of coating thickness, homogeneity, porosity, hardness, grain size and adherence to substrate. Fig. 6 shows SEM photographs of worn HSS pin tips coated with TiCN and TiAlN from supplier A. The density of pores in the TiCN coating is superior to the TiAlN. Porisity in the TiCN coating from supplier B is greater than supplier A as can be seen in Fig. 7. In addition, the disk material has also a clear influence on the pin wear resistance as seen in Fig. 5. The hardest material, steel disk 2, provides the most severe pin wear as expected when compared to the softer steel disk 1.
The wear performance of the coated hard metal can be analyzed in Fig. 8. As expected, the TiAlN coating on the hard metal has increased its wear resistance. The rate of lost volume is substantially reduced for the hard metal coated when compared with WC hard metal without coating. Although the layer is very small, about 4 m, the benefit to increase tool life is clear. The influence of the normal load, and consequently the nominal contact pressure, can also be evaluated in Fig. 8. The rate of lost volume increases with the normal load. However, again the rate decrease with the sliding distance possibly due to the decrease in the nominal contact pressure at the pin surface. The quality of the TiAlN coating in relation to the supplier can be estimated in Fig. 9. The coating quality from supplier B is clearly superior to that from supplier A, even although the TiAlN multilayer process was used by supplier A. In fact, the coating quality was diminished due to the occurrence of porosity as can be seen in Fig. 10.
568
J.D. Bressan et al. / Wear 250 (2001) 561–568
4. Concluding remarks Summarizing, from the analysis of the experimental results in the present investigations the following conclusions can be drawn about the wear performance of the tested PVD coatings and tool materials. Pin on disk tests, micro-hardness test and SEM observations provided important insights into the wear behavior of M2 high speed steel, WC hard metal and coatings. A significant increase in the wear resistance of M2 high speed steel and WC hard metal when coated with TiAlN and TiCN was observed in comparison to the same material without coating, under a surface hertz contact pressure about 800 MPa for HSS and 1000 MPa for hard metal. This can certainly be exploited to increase tool life and decrease tooling costs. The temperatures attained close to the contact surface between pin and disk during the tests are low: about 45◦ C for the load 19.5 N and 60◦ C with the load 29.5 N, for sliding velocity of 0.5 m/s. Thus, it is supposed that the contact temperature has not influenced the wear mechanisms. The quality of the PVD coatings TiAlN and TiCN depend upon the supplier, as it can vary from one to another. The presence of pores in the coating can substantially decrease the wear resistance as was observed with TiCN using the scanning electron microscopy. Excessive porosity has diminished the TiAlN coating wear resistance from one supplier. However, in general the tribological performance of TiAlN is superior to TiCN. The wear rate of the pin, coated or not, depended on the disk material microstructure: grain size, inclusions and
hardness. Similar chemical compositions of electric steel disks led to different pin wear rates. The most severe pin wear rate was from sliding against the hardest steel disk that had particles inside its grain microstructure. Acknowledgements The authors would like to gratefully acknowledge the financial support received from EMBRACO of Joinville, Brazil, CNPq and UDESC-SC, Brazil. References [1] I.M. Hutchings, Tribology: friction and wear of engineering materials, Arnold, Pans, 1995. [2] F.P. Bowden, D. Tabor, Mechanism of metallic friction, Nature 150 (1942) 197–199. [3] J.A. Greenwood, J.B.P. Williamson, The contact of nominally flat surfaces, Proc. R. Soc. London, Ser. A 295 (1966) 300. [4] J.A. Williams, Engineering Tribology, Oxford University Press, Oxford, 1994. [5] K.L. Johnson, Contact Mechanics, Cambridge University Press, Cambridge, UK, 1985. [6] D.A. Hills, D. Nowell, A. Sackfield, Mechanics of Elastic Contacts, Butterworths, London, 1993. [7] J.M. Challen, P.L.B. Oxley, An explanation of the different regimes of friction and wear using asperity deformation models, Wear 53 (1979) 229–243. [8] J.D. Bressan, J.A. Williams, in: Proceedings of 13◦ CBECIMAT on Tribologia: Modelamento Matemático e Mapas de Atrito e Desgaste Abrasivo de Metais, Curitiba, Brazil, 1998. [9] A.A. Torrance, I. Galligan, G. Liraut, A model of friction of a smooth hard surface sliding over a softer one, Wear 212 (1997) 213–220.
Abrasive wear behavior of high speed steel and hard metal coated with TiAlN and TiCN J.D. Bressan a,∗ , R. Hesse a , E.M. Silva Jr. b a
Departmento de Engenharia Mecânica, Centro de Ciências Tecnológicas — CCT/UDESC Campus Universitário, 89223-100 Joinville/SC, Brazil b Fabrica de Componentes, B1. 14 EMBRACO, Rua Rui Barbosa 1020, Cx. P. 91 89.219-901 Joinville/SC, Brazil
Abstract The wear behavior of M2 high speed HSS steel and WC hard metal coated with TiAlN and TiCN were investigated and compared, using the pin on disk standard test with different loads. The coating PVD process has been done by two different suppliers, using an industrial equipment unit with optimized conditions. The coated layers were measured and characterized. The load, sliding distance and velocity of 0.5 m/s were kept constant during the abrasion test in order to control these variables. The counterface disks used were electric steel sheets from three different suppliers. The lost volume and temperature at the pin end have been measured during the wear test. Comparisons of tribological performance for the coated HSS and hard metal were done, using a plot of lost volume versus sliding distance for substrates and coatings. The pin worn surfaces were observed using a scanning electron microscope. A significant increase in the wear resistance of M2 steel and WC hard metal when coated with TiAlN and TiCN was observed. Quality of these coatings depended upon the supplier. Excessive porosity has diminished the TiAlN counting wear resistance from one supplier. However, in general the performance of TiAlN is superior to TiCN. The pin wear rate depended on the disk microstructure. © 2001 Elsevier Science B.V. All rights reserved. Keywords: High speed steel; Hard metal; TiAlN; TiCN; Wear
1. Introduction The classes of processes for surface modification or coating aimed at improving performance by obtaining a good combination of surface and bulk properties not attainable by other means, is known as surface engineering [1]. The two main objectives of surface engineering for tribological applications in components and tooling are: increase wear resistance and modify friction behavior. In some cases both aims are attained. Recently, the industrial and medical applications of wear resistant materials and coatings as well as their economic implications have received much attention from researchers and institutes. It has been of great concern to advance the properties of existing materials and to develop new materials capable of giving better in-service performance of components to the designed functions. Many new wear resistant materials have been presented for quite different applications, but increase in wear resistance has been attained with great efficiency by various coatings and surface treatment techniques.
∗ Corresponding author. E-mail addresses: [email protected] (J.D. Bressan), eraclito m silva [email protected] (E.M. Silva Jr.).
Wear of machinery parts and tooling has direct influence on productivity, efficiency, reliability and quality of manufactured equipment and products. Therefore, it has been an issue of great concern due to tool life, components repair, failure and undesirable production line stops. Besides, future tendency is to increase the speed of production and use of lighter materials. Consequently, velocity and stress levels of machinery and tools will increase and so the wear problems will increase also. Wear is a complex surface phenomenon that occurs mainly due to sliding and impact of hard particles against the solid surfaces, and corrosion, even in the presence of lubricants. It plays an important role in the life of tools and machine components due to the loss of mass of the material, fatigue and failure problems arising from the increasing surface roughness and cracks. Friction arises from the interaction of microscopic asperities or roughness found in all solid surfaces [2]. The surface interactions are mechanical deformation and chemical adhesion of asperities leading to high forces necessary to promote sliding. The effective contact area between two solids is usually a small part of the nominal area and the contact pressure at the level of the roughness, in general, can be much greater than the elastic limit of the material which leads to plastic deformation.
0043-1648/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 6 3 8 - X
562
J.D. Bressan et al. / Wear 250 (2001) 561–568
Wear mechanisms that cause material removal from the surface include: ploughing, wedge formation, micro-chip formation, chemical adhesion, erosion, corrosion, fretting, gouging, scuffing, galling and delamination [1–4]. However, the abrasive wear mechanisms involving plastic deformation mechanisms [4–8] are: ploughing, wedge formation and micro-cutting. In the ploughing mode, a plastic wave is pushed continuously in front of the hard particle and no material is removed. In the wedge formation mechanism, there is the development of a wedge or built-up-edge in front of the sliding hard particle that eventually is sheared off and another wedge is formed. On the other hand, in the cutting mode, formation of a continuous chip in front of the sliding asperity or particle occurs. Both wedge formation and cutting promotes material removal. Numerical methods have also been used to simulate friction and wear [9]. Despite these recent developments in friction and wear theories, which can predict reasonably well the main wear mechanism, and advanced manufacturing techniques, there are still component wear losses due to the lack of design optimization and the presence of contaminant particles from the environment. In abrasion there are the inevitable asperities contacts and these hard particles contaminants. Thus, to reduce wear, it is necessary to increase the surface hardness of components and tooling to values greater than, or close to, the hardness of these hard particles. However, for some applications, wear resistant bulk material may not be appropriate due to cost, weight, other mechanical properties or manufacturing difficulties. For such situations, surface engineering methods can be employed either to apply a hard coating on a substrate with good bulk properties, or to modify the surface properties by changing the surface material phase or chemical composition. Surface engineering processes for tribological applications in components and tooling are therefore necessary in order to increase the material wear resistance or to modify, its friction behavior. The surface and bulk of a material component often need to satisfy different requirements. Better overall performance can be obtained if the surface struc-
ture and properties can be modified, without damaging the underlying bulk material or substrate. The available surface treatment processes should be considered as an integral part of the design and material selection. Costs may be reduced by selecting a cheaper alloy for bulk properties and enhancing only the surface. The surface engineering methods can be classified in two broad classes [1]: surface modification and surface coating. In the first case, the material surface microstructure can be modified selectively without changing its composition as in the quenching process or in the laser surface melting, followed by fast solidification process. Alternatively, composition and microstructure can be modified simultaneously. This can be attained by increasing the thermal diffusion of a chemical compound into the surface. The composition and micro-structural changes which can be attained by these methods are obviously limited, and for may purpose film deposition or surface coating can be used. Some techniques are only applicable to metals and some exclusively to steels, although others can be applied to polymers and ceramics. A comparative study of the wear behavior of PVD coatings TiAlN and TiCN were carried out in the present work. The coating process has been done by two different suppliers, using an industrial equipment unit under maximized conditions. The wear performance was experimentally investigated using the pin on disk method. The pins were coated HSS steel and hard metal WC, and the disks were made from electric steel sheets.
2. Materials and experimental procedure PVD coating of the pin surfaces processes was carried out in industrial equipment at a specialized company: coating suppliers A and B. The nominal characteristics of the deposited coatings are shown in Table 1. The hardness of the coatings and the substrate materials have been experimentally measured and the values are seen in Table 2.
Table 1 Nominal characteristics of the deposited TiAlN and TiCN coatings given by suppliers Coating
Microhardness (HV 0.05)
Thickness of coating (m)
Process temperature (◦ C)
Friction coefficient against steel ()
TiCN TiAlN TiAlN × multilayer
3000 3500 3000
1–4 1–3 1–5
≤500 ≤550 ≤550
0.4 0.4 0.4
Table 2 Experimental microhardness results for pin materials substrates and the deposited TiAlN and TiCN coatings from suppliers A and B Material\coating
Microhardness (HV 0.5)
HSS
775
WC
1370
HSS\TiCN
HSS\TiAlN
WC\TiAlN
A
B
A
B
A
B
836
1015
940
–
1730
1675
J.D. Bressan et al. / Wear 250 (2001) 561–568
563
for the hard metal. The pin sliding velocity during the test was kept constant at about 0.5 m/s and the radius of circular wear track was 16 mm. The counterface disks were tested as received and were made of 0.5 mm electric steel sheets received from three different suppliers: steel 1, steel 2 and steel 3 with Vickers hardness 103, 156 and 144 kg/mm2 , respectively. These disks, 62 mm in diameter, were glued to a steel disk base which was held by a chuck jaw and was driven by a small electric motor. The materials were tested by couples and were submitted to similar nominal abrasive conditions. The lost volume of the pin and disk materials were calculated by its measured mass variations. Every 200 m of sliding distance the test was automatically interrupted and the mass variation of pin and disk were
Fig. 1. Pin geometry.
The HSS steel grade M2 pin material was received from Villares of Brazil with chemical composition: 0.85% C; 0.30% Mn; 0.25% Si; 2% V; 4% Cr; 6% W; 5% Mo. The WC hard metal alloy pin material had 12% Co and grain size varying from 1 to 2 m. The electric steel disk materials had chemical composition: 0.10% C; 0.20% Al; 2.1% Si; 0.15% Mn; 0.04% P; 0.008% S. The wear test were run in a pin on disk laboratory equipment with constant load and velocity, according to the ASTM G-99-95 standard, and under normal atmosphere conditions of about 20◦ C and 54–60% relative humidity. The pin contact ends had a curvature radius of 10 mm and were milled to give good surface finish (Fig. 1). The applied normal load were 19.5 and 29.5 N corresponding to a maximum Hertzian contact pressure [5] of approximately 760 and 870 MPa for the HSS steel, and 970 and 1120 MPa
Fig. 2. Mean temperature at the pin end for HSS under normal loads of 19.5 and 29.5 N.
Fig. 3. Microstructure of the electric steel disk surfaces, showing grain size, and inclusion, particles and Vickers hardness.
564
J.D. Bressan et al. / Wear 250 (2001) 561–568
Fig. 4. Comparisons of rate of lost mass for pins under the normal load of 38.6 N, sliding against steel disks 1 and 2.
measured using an analytic balance accurate to 0.1 mg. The lost volume of the pins was calculated by dividing the measured lost mass by the pin material density: 7.85 g/cm3 for HSS steel and 15.7 g/cm3 for WC hard metal. Temperature of the pin contact end was measured during the tests, using a thermocouple inserted at a hole in the pin tip end. The temperature point was 2 mm from the disk contact surface. The maximum temperature attained during the test has been registered and is commented next.
3. Wear behavior and results The experimental results obtained from the pin on disk tests were plotted to show temperature and the lost volume versus the sliding distance for constant velocity and normal load. In all cases there were the formation of scratches on the steel disk after the first 200 m sliding distance. In Fig. 2, the mean temperatures attained during the wear test at the pin end is shown. The point of measurement is
about 2 mm from the contact surface. Temperature rises as the load increases, but are below 70◦ C for HSS and WC. The disk microstructures are different as can be seen in Fig. 3, although they are similar electric steels. The grain size varies quite a lot and hard particles are possibly present in the grain. In Fig. 4, a comparison of the lost mass rate for HSS, HSS coated with TiCN (HSS\TiCN) and WC pins are plotted as functions of the sliding distance. The benefit of the TiCN coating is very clear. The rate of lost mass decreases with the sliding distance possibly due to the decrease in the nominal surface contact pressure as the contact channel in the disk widened. The wear resistance comparison between HSS\TiCN and the hard metal WC, in terms of pin lost mass is misleading as the tool life is related more properly to the lost volume at its edges. The density of the hard metal is double the density of the HSS. Thus, the lost volume for the hard metal is less than the HSS for the same amount of lost mass. In fact, the hard metal lost volume is half the lost volume of the HSS. Therefore, the lost volume is more
Fig. 5. Pin’s lost volume vs. sliding distance for HSS coated with TiCN from suppliers A and B. Normal load of 29.5 N is indicated by II.
J.D. Bressan et al. / Wear 250 (2001) 561–568
565
Fig. 6. (a) and (b) Pin’s lost volume vs. sliding distance. WC coated with TiAlN from suppliers B and WC without coating. Normal load of 19.5 N (I) and 29.5 N (II).
Fig. 7. Pin’s lost volume vs. sliding distance. WC coated with TiAlN from suppliers B and WC coated with TiAlN × multilayer from supplier A. Normal load of 19.5 N (I).
566
J.D. Bressan et al. / Wear 250 (2001) 561–568
Fig. 8. Scanning electron microscope photographs of HSS pin tips coated with TiAlN and TiCN from supplier A, after sliding 2400 m over steel disks 1, 2 and 3. Normal load of 19.5 N.
Fig. 9. Scanning electron microscope photographs of WC pin tips coated with TiAlN multilayers on hard metal pin tips, after sliding 200 m over steel disks 1, 2 and 3. Normal load of 29.5 N. Porosity and hard particles and present at the coating. Abrasive scratches are also observed. Coating supplier A.
J.D. Bressan et al. / Wear 250 (2001) 561–568
567
Fig. 10. Scanning electron microscope photographs of HSS pin tips coated with TiCN after sliding 2000 m over steel disks 1, 2 and 3. Normal load of 19.5 N. Coating process from supplier B.
appropriate to measure the wear resistance of the pin materials and coatings, and is used in the following graphs. In Fig. 5, a comparison of the wear performance of the HSS coated with TiCN from suppliers A and B is made. In general, the performance of the coating from company B is better than the coating delivered by company A, although the nominal specifications are the same. This is possibly due to variations of coating thickness, homogeneity, porosity, hardness, grain size and adherence to substrate. Fig. 6 shows SEM photographs of worn HSS pin tips coated with TiCN and TiAlN from supplier A. The density of pores in the TiCN coating is superior to the TiAlN. Porisity in the TiCN coating from supplier B is greater than supplier A as can be seen in Fig. 7. In addition, the disk material has also a clear influence on the pin wear resistance as seen in Fig. 5. The hardest material, steel disk 2, provides the most severe pin wear as expected when compared to the softer steel disk 1.
The wear performance of the coated hard metal can be analyzed in Fig. 8. As expected, the TiAlN coating on the hard metal has increased its wear resistance. The rate of lost volume is substantially reduced for the hard metal coated when compared with WC hard metal without coating. Although the layer is very small, about 4 m, the benefit to increase tool life is clear. The influence of the normal load, and consequently the nominal contact pressure, can also be evaluated in Fig. 8. The rate of lost volume increases with the normal load. However, again the rate decrease with the sliding distance possibly due to the decrease in the nominal contact pressure at the pin surface. The quality of the TiAlN coating in relation to the supplier can be estimated in Fig. 9. The coating quality from supplier B is clearly superior to that from supplier A, even although the TiAlN multilayer process was used by supplier A. In fact, the coating quality was diminished due to the occurrence of porosity as can be seen in Fig. 10.
568
J.D. Bressan et al. / Wear 250 (2001) 561–568
4. Concluding remarks Summarizing, from the analysis of the experimental results in the present investigations the following conclusions can be drawn about the wear performance of the tested PVD coatings and tool materials. Pin on disk tests, micro-hardness test and SEM observations provided important insights into the wear behavior of M2 high speed steel, WC hard metal and coatings. A significant increase in the wear resistance of M2 high speed steel and WC hard metal when coated with TiAlN and TiCN was observed in comparison to the same material without coating, under a surface hertz contact pressure about 800 MPa for HSS and 1000 MPa for hard metal. This can certainly be exploited to increase tool life and decrease tooling costs. The temperatures attained close to the contact surface between pin and disk during the tests are low: about 45◦ C for the load 19.5 N and 60◦ C with the load 29.5 N, for sliding velocity of 0.5 m/s. Thus, it is supposed that the contact temperature has not influenced the wear mechanisms. The quality of the PVD coatings TiAlN and TiCN depend upon the supplier, as it can vary from one to another. The presence of pores in the coating can substantially decrease the wear resistance as was observed with TiCN using the scanning electron microscopy. Excessive porosity has diminished the TiAlN coating wear resistance from one supplier. However, in general the tribological performance of TiAlN is superior to TiCN. The wear rate of the pin, coated or not, depended on the disk material microstructure: grain size, inclusions and
hardness. Similar chemical compositions of electric steel disks led to different pin wear rates. The most severe pin wear rate was from sliding against the hardest steel disk that had particles inside its grain microstructure. Acknowledgements The authors would like to gratefully acknowledge the financial support received from EMBRACO of Joinville, Brazil, CNPq and UDESC-SC, Brazil. References [1] I.M. Hutchings, Tribology: friction and wear of engineering materials, Arnold, Pans, 1995. [2] F.P. Bowden, D. Tabor, Mechanism of metallic friction, Nature 150 (1942) 197–199. [3] J.A. Greenwood, J.B.P. Williamson, The contact of nominally flat surfaces, Proc. R. Soc. London, Ser. A 295 (1966) 300. [4] J.A. Williams, Engineering Tribology, Oxford University Press, Oxford, 1994. [5] K.L. Johnson, Contact Mechanics, Cambridge University Press, Cambridge, UK, 1985. [6] D.A. Hills, D. Nowell, A. Sackfield, Mechanics of Elastic Contacts, Butterworths, London, 1993. [7] J.M. Challen, P.L.B. Oxley, An explanation of the different regimes of friction and wear using asperity deformation models, Wear 53 (1979) 229–243. [8] J.D. Bressan, J.A. Williams, in: Proceedings of 13◦ CBECIMAT on Tribologia: Modelamento Matemático e Mapas de Atrito e Desgaste Abrasivo de Metais, Curitiba, Brazil, 1998. [9] A.A. Torrance, I. Galligan, G. Liraut, A model of friction of a smooth hard surface sliding over a softer one, Wear 212 (1997) 213–220.