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Ion implantation boriding of iron and AISI M2 steel using a high-current density, low energy, broad-beam ion source
Surface and Coatings Technology 103–104 (1998) 52–57
Ion implantation boriding of iron and AISI M2 steel using a high-current density, low energy, broad-beam ion source J.A. Davis a, P.J. Wilbur a,*, D.L. Williamson b, R. Wei c, J.J. Vajo c a Department of Mechanical Engineering, Colorado State University, Fort Collins, CO 80523, USA b Physics Department, Colorado School of Mines, Golden, CO 80401, USA c Hughes Research Laboratories, Malibu, CA 90625, USA
Abstract Ions derived from solid boron and extracted into a broad beam were implanted at low energies into a-iron and AISI M2 tool steel surfaces to alter their tribological characteristics. The implantations were accomplished at elevated temperatures and this facilitated thermal diffusion yielding relatively thick treated layers. The effects of the surface temperature associated with the implantation process on sliding wear behavior, hardness, microstructure and B concentration profiles were studied systematically. Boron implantation at 20 keV and 500 mA cm−2 reduced the sliding wear rate of iron and quenched-and-tempered M2 steel at all temperatures studied. Implantation temperatures as low as 600 °C were sufficient to effect significant thermal diffusion leading to a thick wear resistant layer in iron, while 700 °C was needed for the steel. © 1998 Elsevier Science S.A. Keywords: Ion implantation; Boriding; Diffusion; Boron
1. Introduction Conventional boriding [1,2] has been accomplished in various ways (i.e. pack, bath, gas and plasma boriding) to create wear resistant iron boride layers on the surfaces of iron and steels. These layers are thick (10–150 mm) but the processing requires large amounts of thermal energy for the pack and bath methods and toxic and explosive gases for the gas and plasma methods [1]. Another drawback of boriding involves an inability to separate the delivery rate of boron to a surface and the diffusion rate of boron in the metal beneath the surface, because both are controlled by a single system temperature. This leads to the formation of dual boride layers (FeB over Fe B) in which opposing residual 2 stresses can induce cracking along the interface between layers and attendant spalling leading to rapid wear of the system. A process in which the boron delivery rate is controlled independent from the diffusion rate is conventional ion implantation. Typically, however, boron has been implanted at low sample temperatures and low doses (up to 2×1017 B cm−2) and this has yielded thin * Corresponding author. 0257-8972/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 37 4 - 0
amorphous layers [3,4]. While these layers can reduce the wear rate of a system [3] they tend to wear away quickly at higher loads. Further, systems used to do this implanting have the drawback of using the same toxic and explosive gases used for gas and plasma boriding, although in much lower amounts. To alleviate the problems of toxic gases, low dose rates and low diffusion rates, a system which vaporizes boron from a solid, ionizes the boron vapor and extracts an ion beam with a high beam current density has been used [5]. Implantation results obtained with this system at high substrate temperatures (600–950 °C ), a high beam current density (500 mA cm−2) and a relatively low beam energy (20 keV ) into a-iron and AISI M2 are described herein. A goal is to determine the extent to which boron delivered to ferrous surfaces under these conditions diffuses beyond the ballistic implantation depth, forms desired borides and decreases surface wear rates.
2. Experimental Pure a-iron (99.5%) and M2 steel were used to investigate ion implantation of boron at high substrate temper-
J.A. Davis et al. / Surface and Coatings Technology 103–104 (1998) 52–57
atures, that is, ion implantation boriding. The iron discs were 4.8 cm in diameter and 1.83 mm thick and had been polished to a mean roughness of 0.015 mm. The M2 steel (4.4 Cr, 3.0 Mo, 2.3 V, 1.9 W, 4.1 C, balance Fe in at%) samples were blocks machined to the specifications for block-on-ring testing [6 ], heat treated to a quenched and tempered state (R 62.5), and polished to c a final mean roughness of 0.25 mm. They were worn against unimplanted 35-mm-diameter rings fabricated from the same material and heat treated to the same state as the blocks but with a surface roughness of 0.3 mm. Discs and blocks were cleaned ultrasonically for 5 min each in chlorothene and acetone before they were placed in a vacuum system and implantation borided. The metal ion implantation system [5] used to do this derived boron from a pure amorphous powder (99.99%, 300 mesh). Implantation involved electron bombardment heating of 1-cm-diameter boron pellets in a crucible until the boron was vaporized and then ionization to create a pure boron plasma. A beam of boron ions extracted electrostatically from this plasma was then implanted directly into a disc or block. During processing, the temperature of the disc or block was monitored by a thermocouple near the surface and controlled by adjusting the power to a carbon resistance heater on which the disc or block was mounted. To determine the changes in surface microstructure induced by ion implantation boriding, the surfaces of both the iron discs and M2 blocks were analyzed using X-ray diffraction ( XRD) utilizing Cu Ka (8 keV ) X-rays in the Bragg–Brentano configuration. Such X-rays sample surface layers that are 1–3 mm thick. In some cases conversion electron Mo¨ssbauer spectroscopy (CEMS) was used to characterize a 0.1-mm-thick surface layer on a-Fe discs. Boron concentrations measured as a function of depth using Auger electron spectroscopy
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(AES) coupled with ion sputtering were used to determine treated-layer thicknesses. Sensitivity factors for boron and iron in the AES system were determined by analyzing a sample of 99% pure FeB. The iron discs were wear tested on an oscillating pinon-disk machine [7] to determine the effect of ion implantation boriding on their wear characteristics. A WC pin with a tip radius of 3.2 mm, a load of 3.5 N (Hertzian contact stress of 1.7 GPa), a sliding speed of 100 rpm (~0.17 m s−1) and continuous washing with a boundary lubricant (10% oleic acid in kerosene) were used in these tests. Wear rates were determined by removing the disc and measuring its mass periodically over a 30-h total test duration. Mean masses, each based on ten measurements on a balance accurate to ±0.05 mg, were used to determine mass losses and wear rates. The M2 steel blocks were tested using a block-onring tribometer for 1 h at 300 rpm (0.55 m s−1) and a normal load of 222 N (271 MPa Hertzian stress). These tests were conducted with a water-soluble boundary lubricant (Long-life 20/20@ diluted 20-to-1 with water). The volume worn from each block was determined using three wear-scar-width measurements [6 ] and verified by associated mass losses.
3. Results 3.1. Iron Since temperature has been found to have a dominant influence on treated-layer thickness and as a consequence on tribological performance, its effect was studied. Iron samples were B ion implanted at 20 keV and 500 mA cm−2 at temperatures between 600 and 950 °C. The data of Fig. 1 show high concentrations of
Fig. 1. Effect of temperature on boron concentration profiles for implantation-borided iron.
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J.A. Davis et al. / Surface and Coatings Technology 103–104 (1998) 52–57
ballistic depth of 0.05 mm [9]. It is noted that substantial boron diffusion beyond 0.8 mm may be occurring, but it is not certain from the data of Fig. 1 because noise obscures the boron AES signal at concentrations below ~10 at%. The XRD data of Fig. 2 show the dominant boride is Fe B at all temperatures and this is consistent with 2 the CEMS data. At some temperatures Fe B is also 23 6 observed and at 950 °C, Fe B begins to appear. At no 3 conditions, however, is the undesirable, brittle FeB phase observed. Mass-loss histories over 30-h pin-on-disc wear tests, which are shown in Fig. 3, indicate that ion implantation boriding at any temperature improves the wear resistance of iron. The discs implanted at temperatures of 750 °C and below, which had the continuous boride layers, however, show little to no mass loss, while those implanted at the higher temperatures where the precipitate coverage is discontinuous, exhibit substantial mass loss rates. Surface micrographs taken during wear testing suggest the increased wear on the 800–900 °C discs occurs as precipitates are pushed into the iron matrix thereby exposing it to increased pin contact. This did not occur on the continuous layers produced at the lower temperatures. Fig. 2. XRD patterns for implantation-borided iron.
3.2. AISI M2 boron (~39 at%) are produced near the surface at temperatures of 750 °C and below. CEMS analyses on the 600 and 700 °C discs showed they were 89 and 93% Fe B, respectively, near their surfaces and this is consis2 tent with both the data of Fig. 1 and the presence of a continuous Fe B layer. At higher temperatures (800 °C 2 and above) Fig. 1 shows mean surface boron concentrations at and below 29 at%. Because the AES beam diameter for these measurements is 100 mm they represent mean concentrations over a relatively large area. Corresponding CEMS data yielded 62 and 72% Fe B in 2 a-Fe, respectively, at 800 and 900 °C. These results suggest that the surface layers become discontinuous at the higher temperatures. Optical microscopy reinforced these results by revealing continuous boride layers on the samples implanted at and below 750 °C and discrete micron-size boride precipitates in an iron matrix above this temperature. These precipitates have been seen previously and identified as Fe B [8]. It is noted that 2 the theoretical concentration of boron in Fe B is 33 at% 2 and it is suggested that the additional 6% B associated with the 600–750 °C discs of Fig. 1 is due to diffusing boron that is trapped as a consequence of the rapid cooling rate (65 °C min−1) that occurs when the implantation stops. The AES concentration profiles also reveal that the layer thicknesses, which range from 0.55 to ~0.8 mm, are an order of magnitude greater than the calculated
Ion implantation boriding of M2 tool steel in the temperature range 600–900 °C was also studied. Fig. 4 shows the B concentration profile determine by AES for a block that was implantation borided at 700 °C. It shows that boron diffuses to a depth (~0.5 mm) that is much greater than the calculated ballistic depth of 0.05 mm [9]. The boron diffusion depth in the steel is less than that for pure iron at this temperature probably because it contains alloying elements, which have been reported to slow the diffusion of B in steels [10]. XRD measurements show no evidence of borides on M2 steel blocks implanted at 500 and 600 °C, Fe B 23 6 with small amounts of Fe B at 700 and 750 °C and 2 Fe B with a small amount of Fe B at 900 °C. No 23 6 3 evidence of the brittle FeB phase was seen in any block. On the basis of these tests, it appears that B diffusion in M2 steel is insignificant for temperatures at and below 600 °C. The data of Fig. 5 show that borides formed in implantation borided M2 steel increase its surface hardness relative to M2 steel that has experienced the same thermal history. This figure suggests, however, that the high temperatures required for implantation boriding induce a substantial softening of the bulk material that can overwhelm the hardening of the surface. The data indicate that substantial softening of the bulk material occurs during the 15-min processing time at temperatures of 700 °C and above. However, there is evidence
J.A. Davis et al. / Surface and Coatings Technology 103–104 (1998) 52–57
Fig. 3. Effect of implantation boriding temperature on iron wear behavior.
Fig. 4. Boron concentration profile for implantation-borided M2 tool steel.
Fig. 5. Effect of implantation boriding temperature on surface hardness of quenched-and-tempered M2 steel.
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Fig. 6. Effect of implantation boriding temperature on wear rate of quenched-and-tempered M2 steel.
of some bulk hardness recovery for the block processed at 900 °C. This probably occurs because 900 °C is above the ferrite-to-austenite transition temperatures (820– 880 °C ) and a sufficiently rapid cool down from this temperature enables reformation of sufficient martensite to recover some of the hardness. Evidence that implantation boriding improves the wear resistance of a surface is shown in Fig. 6. This figure compares wear rates of surfaces that have been implantation borided to those for surfaces that have experienced corresponding implantation boriding thermal histories without the boron. It shows the boron reduces the wear rate (up to 27% at 700 °C ). However, the annealing associated with the processing, which was seen in Fig. 5 to lower bulk hardness, has the additional detrimental effect of increasing the wear rate above that for the untreated steel. The effects of reforming martensite by heating above 900 °C and quenching are also apparent in this figure. They are reflected in the recovery of improved wear behavior associated with the increase in implantation temperature from 800 to 900 °C. Although none of these surfaces which were implantation borided after heat treatment are as wear resistant as the untreated one, implantation boriding in conjunction with or before heat treatment could yield surfaces that are more wear resistant.
implantation depth. Temperatures above 700 °C are required to produce similar diffusion behavior in M2 steel. At an implantation current density of 0.5 mA cm−2, the dominant boride produced in a-Fe over the 600–950 °C temperature range is Fe B but low concen2 trations of Fe B , and at the highest temperature 23 6 Fe B, are also observed. Under similar processing condi3 tions, Fe B is the dominant phase produced in M2 23 6 steel. Under the conditions investigated, the brittle FeB phase is not observed. The continuous boride layers that formed on a-Fe substrates at temperatures <750 °C are much more wear resistant than the boride discontinuous precipitates which form in an iron matrix at higher temperatures. A borided M2 steel surface is more wear resistant than one that has seen the thermal history associated with implantation boriding, but neither is as wear resistant as the quenched-and-tempered material.
4. Conclusions
References
An energetic, high-current-density, broad boron ion beam can be extracted from a pure boron plasma sustained in an ion source by bombarding solid boron with electrons that vaporize it and then ionize the vaporized atoms. Boron ions implanted into a-Fe surfaces held at temperatures above 600 °C for 15 min diffuse over an order of magnitude beyond the ballistic
Acknowledgement This work was supported by the National Science Foundation under Grant No. CMS-9414459.
[1] A.G. von Matuschka, Boronizing, Heyden & Sons, Philadelphia, 1980. [2] F. Miyashita, K. Yokota, Surf. Coat. Technol. 84 (1996) 334. [3] H.J. Kim, W.B. Carter, R.F. Hochman, E.I. Meletis, Mater. Sci. Eng. 69 (1985) 297. [4] N. Millard, G. Marest, N. Moncoffre, J. Tousset, Surf. Coat. Technol. 51 (1992) 446. [5] P.J. Wilbur, R. Wei, Rev. Scient. Instrum. 63 (4) (1992) 2491.
J.A. Davis et al. / Surface and Coatings Technology 103–104 (1998) 52–57 [6 ] ASTM, Annual Book of ASTM Standards, Designation G77-83, v. 03.02, ASTM, Philadelphia, PA, 1983, p. 453. [7] R. Wei, P.J. Wilbur, W.S. Sampath, D.L. Williamson, Y. Qu, L. Wong, J. Tribology 112 (1990) 27–36. [8] P.J. Wilbur, J.A. Davis, D.L. Williamson, J.J. Vajo, R. Wei, Surf. Coat. Technol., 96 (1997) 52–57.
57
[9] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Vol. 1, Pergamon, New York, 1985. [10] G.V. Sampsonov, A.P. Epik, in: H.H. Hausner ( Ed.), Coatings of High Temperature Materials, Plenum Press, New York, 1966, p. 2.
Ion implantation boriding of iron and AISI M2 steel using a high-current density, low energy, broad-beam ion source J.A. Davis a, P.J. Wilbur a,*, D.L. Williamson b, R. Wei c, J.J. Vajo c a Department of Mechanical Engineering, Colorado State University, Fort Collins, CO 80523, USA b Physics Department, Colorado School of Mines, Golden, CO 80401, USA c Hughes Research Laboratories, Malibu, CA 90625, USA
Abstract Ions derived from solid boron and extracted into a broad beam were implanted at low energies into a-iron and AISI M2 tool steel surfaces to alter their tribological characteristics. The implantations were accomplished at elevated temperatures and this facilitated thermal diffusion yielding relatively thick treated layers. The effects of the surface temperature associated with the implantation process on sliding wear behavior, hardness, microstructure and B concentration profiles were studied systematically. Boron implantation at 20 keV and 500 mA cm−2 reduced the sliding wear rate of iron and quenched-and-tempered M2 steel at all temperatures studied. Implantation temperatures as low as 600 °C were sufficient to effect significant thermal diffusion leading to a thick wear resistant layer in iron, while 700 °C was needed for the steel. © 1998 Elsevier Science S.A. Keywords: Ion implantation; Boriding; Diffusion; Boron
1. Introduction Conventional boriding [1,2] has been accomplished in various ways (i.e. pack, bath, gas and plasma boriding) to create wear resistant iron boride layers on the surfaces of iron and steels. These layers are thick (10–150 mm) but the processing requires large amounts of thermal energy for the pack and bath methods and toxic and explosive gases for the gas and plasma methods [1]. Another drawback of boriding involves an inability to separate the delivery rate of boron to a surface and the diffusion rate of boron in the metal beneath the surface, because both are controlled by a single system temperature. This leads to the formation of dual boride layers (FeB over Fe B) in which opposing residual 2 stresses can induce cracking along the interface between layers and attendant spalling leading to rapid wear of the system. A process in which the boron delivery rate is controlled independent from the diffusion rate is conventional ion implantation. Typically, however, boron has been implanted at low sample temperatures and low doses (up to 2×1017 B cm−2) and this has yielded thin * Corresponding author. 0257-8972/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 37 4 - 0
amorphous layers [3,4]. While these layers can reduce the wear rate of a system [3] they tend to wear away quickly at higher loads. Further, systems used to do this implanting have the drawback of using the same toxic and explosive gases used for gas and plasma boriding, although in much lower amounts. To alleviate the problems of toxic gases, low dose rates and low diffusion rates, a system which vaporizes boron from a solid, ionizes the boron vapor and extracts an ion beam with a high beam current density has been used [5]. Implantation results obtained with this system at high substrate temperatures (600–950 °C ), a high beam current density (500 mA cm−2) and a relatively low beam energy (20 keV ) into a-iron and AISI M2 are described herein. A goal is to determine the extent to which boron delivered to ferrous surfaces under these conditions diffuses beyond the ballistic implantation depth, forms desired borides and decreases surface wear rates.
2. Experimental Pure a-iron (99.5%) and M2 steel were used to investigate ion implantation of boron at high substrate temper-
J.A. Davis et al. / Surface and Coatings Technology 103–104 (1998) 52–57
atures, that is, ion implantation boriding. The iron discs were 4.8 cm in diameter and 1.83 mm thick and had been polished to a mean roughness of 0.015 mm. The M2 steel (4.4 Cr, 3.0 Mo, 2.3 V, 1.9 W, 4.1 C, balance Fe in at%) samples were blocks machined to the specifications for block-on-ring testing [6 ], heat treated to a quenched and tempered state (R 62.5), and polished to c a final mean roughness of 0.25 mm. They were worn against unimplanted 35-mm-diameter rings fabricated from the same material and heat treated to the same state as the blocks but with a surface roughness of 0.3 mm. Discs and blocks were cleaned ultrasonically for 5 min each in chlorothene and acetone before they were placed in a vacuum system and implantation borided. The metal ion implantation system [5] used to do this derived boron from a pure amorphous powder (99.99%, 300 mesh). Implantation involved electron bombardment heating of 1-cm-diameter boron pellets in a crucible until the boron was vaporized and then ionization to create a pure boron plasma. A beam of boron ions extracted electrostatically from this plasma was then implanted directly into a disc or block. During processing, the temperature of the disc or block was monitored by a thermocouple near the surface and controlled by adjusting the power to a carbon resistance heater on which the disc or block was mounted. To determine the changes in surface microstructure induced by ion implantation boriding, the surfaces of both the iron discs and M2 blocks were analyzed using X-ray diffraction ( XRD) utilizing Cu Ka (8 keV ) X-rays in the Bragg–Brentano configuration. Such X-rays sample surface layers that are 1–3 mm thick. In some cases conversion electron Mo¨ssbauer spectroscopy (CEMS) was used to characterize a 0.1-mm-thick surface layer on a-Fe discs. Boron concentrations measured as a function of depth using Auger electron spectroscopy
53
(AES) coupled with ion sputtering were used to determine treated-layer thicknesses. Sensitivity factors for boron and iron in the AES system were determined by analyzing a sample of 99% pure FeB. The iron discs were wear tested on an oscillating pinon-disk machine [7] to determine the effect of ion implantation boriding on their wear characteristics. A WC pin with a tip radius of 3.2 mm, a load of 3.5 N (Hertzian contact stress of 1.7 GPa), a sliding speed of 100 rpm (~0.17 m s−1) and continuous washing with a boundary lubricant (10% oleic acid in kerosene) were used in these tests. Wear rates were determined by removing the disc and measuring its mass periodically over a 30-h total test duration. Mean masses, each based on ten measurements on a balance accurate to ±0.05 mg, were used to determine mass losses and wear rates. The M2 steel blocks were tested using a block-onring tribometer for 1 h at 300 rpm (0.55 m s−1) and a normal load of 222 N (271 MPa Hertzian stress). These tests were conducted with a water-soluble boundary lubricant (Long-life 20/20@ diluted 20-to-1 with water). The volume worn from each block was determined using three wear-scar-width measurements [6 ] and verified by associated mass losses.
3. Results 3.1. Iron Since temperature has been found to have a dominant influence on treated-layer thickness and as a consequence on tribological performance, its effect was studied. Iron samples were B ion implanted at 20 keV and 500 mA cm−2 at temperatures between 600 and 950 °C. The data of Fig. 1 show high concentrations of
Fig. 1. Effect of temperature on boron concentration profiles for implantation-borided iron.
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J.A. Davis et al. / Surface and Coatings Technology 103–104 (1998) 52–57
ballistic depth of 0.05 mm [9]. It is noted that substantial boron diffusion beyond 0.8 mm may be occurring, but it is not certain from the data of Fig. 1 because noise obscures the boron AES signal at concentrations below ~10 at%. The XRD data of Fig. 2 show the dominant boride is Fe B at all temperatures and this is consistent with 2 the CEMS data. At some temperatures Fe B is also 23 6 observed and at 950 °C, Fe B begins to appear. At no 3 conditions, however, is the undesirable, brittle FeB phase observed. Mass-loss histories over 30-h pin-on-disc wear tests, which are shown in Fig. 3, indicate that ion implantation boriding at any temperature improves the wear resistance of iron. The discs implanted at temperatures of 750 °C and below, which had the continuous boride layers, however, show little to no mass loss, while those implanted at the higher temperatures where the precipitate coverage is discontinuous, exhibit substantial mass loss rates. Surface micrographs taken during wear testing suggest the increased wear on the 800–900 °C discs occurs as precipitates are pushed into the iron matrix thereby exposing it to increased pin contact. This did not occur on the continuous layers produced at the lower temperatures. Fig. 2. XRD patterns for implantation-borided iron.
3.2. AISI M2 boron (~39 at%) are produced near the surface at temperatures of 750 °C and below. CEMS analyses on the 600 and 700 °C discs showed they were 89 and 93% Fe B, respectively, near their surfaces and this is consis2 tent with both the data of Fig. 1 and the presence of a continuous Fe B layer. At higher temperatures (800 °C 2 and above) Fig. 1 shows mean surface boron concentrations at and below 29 at%. Because the AES beam diameter for these measurements is 100 mm they represent mean concentrations over a relatively large area. Corresponding CEMS data yielded 62 and 72% Fe B in 2 a-Fe, respectively, at 800 and 900 °C. These results suggest that the surface layers become discontinuous at the higher temperatures. Optical microscopy reinforced these results by revealing continuous boride layers on the samples implanted at and below 750 °C and discrete micron-size boride precipitates in an iron matrix above this temperature. These precipitates have been seen previously and identified as Fe B [8]. It is noted that 2 the theoretical concentration of boron in Fe B is 33 at% 2 and it is suggested that the additional 6% B associated with the 600–750 °C discs of Fig. 1 is due to diffusing boron that is trapped as a consequence of the rapid cooling rate (65 °C min−1) that occurs when the implantation stops. The AES concentration profiles also reveal that the layer thicknesses, which range from 0.55 to ~0.8 mm, are an order of magnitude greater than the calculated
Ion implantation boriding of M2 tool steel in the temperature range 600–900 °C was also studied. Fig. 4 shows the B concentration profile determine by AES for a block that was implantation borided at 700 °C. It shows that boron diffuses to a depth (~0.5 mm) that is much greater than the calculated ballistic depth of 0.05 mm [9]. The boron diffusion depth in the steel is less than that for pure iron at this temperature probably because it contains alloying elements, which have been reported to slow the diffusion of B in steels [10]. XRD measurements show no evidence of borides on M2 steel blocks implanted at 500 and 600 °C, Fe B 23 6 with small amounts of Fe B at 700 and 750 °C and 2 Fe B with a small amount of Fe B at 900 °C. No 23 6 3 evidence of the brittle FeB phase was seen in any block. On the basis of these tests, it appears that B diffusion in M2 steel is insignificant for temperatures at and below 600 °C. The data of Fig. 5 show that borides formed in implantation borided M2 steel increase its surface hardness relative to M2 steel that has experienced the same thermal history. This figure suggests, however, that the high temperatures required for implantation boriding induce a substantial softening of the bulk material that can overwhelm the hardening of the surface. The data indicate that substantial softening of the bulk material occurs during the 15-min processing time at temperatures of 700 °C and above. However, there is evidence
J.A. Davis et al. / Surface and Coatings Technology 103–104 (1998) 52–57
Fig. 3. Effect of implantation boriding temperature on iron wear behavior.
Fig. 4. Boron concentration profile for implantation-borided M2 tool steel.
Fig. 5. Effect of implantation boriding temperature on surface hardness of quenched-and-tempered M2 steel.
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Fig. 6. Effect of implantation boriding temperature on wear rate of quenched-and-tempered M2 steel.
of some bulk hardness recovery for the block processed at 900 °C. This probably occurs because 900 °C is above the ferrite-to-austenite transition temperatures (820– 880 °C ) and a sufficiently rapid cool down from this temperature enables reformation of sufficient martensite to recover some of the hardness. Evidence that implantation boriding improves the wear resistance of a surface is shown in Fig. 6. This figure compares wear rates of surfaces that have been implantation borided to those for surfaces that have experienced corresponding implantation boriding thermal histories without the boron. It shows the boron reduces the wear rate (up to 27% at 700 °C ). However, the annealing associated with the processing, which was seen in Fig. 5 to lower bulk hardness, has the additional detrimental effect of increasing the wear rate above that for the untreated steel. The effects of reforming martensite by heating above 900 °C and quenching are also apparent in this figure. They are reflected in the recovery of improved wear behavior associated with the increase in implantation temperature from 800 to 900 °C. Although none of these surfaces which were implantation borided after heat treatment are as wear resistant as the untreated one, implantation boriding in conjunction with or before heat treatment could yield surfaces that are more wear resistant.
implantation depth. Temperatures above 700 °C are required to produce similar diffusion behavior in M2 steel. At an implantation current density of 0.5 mA cm−2, the dominant boride produced in a-Fe over the 600–950 °C temperature range is Fe B but low concen2 trations of Fe B , and at the highest temperature 23 6 Fe B, are also observed. Under similar processing condi3 tions, Fe B is the dominant phase produced in M2 23 6 steel. Under the conditions investigated, the brittle FeB phase is not observed. The continuous boride layers that formed on a-Fe substrates at temperatures <750 °C are much more wear resistant than the boride discontinuous precipitates which form in an iron matrix at higher temperatures. A borided M2 steel surface is more wear resistant than one that has seen the thermal history associated with implantation boriding, but neither is as wear resistant as the quenched-and-tempered material.
4. Conclusions
References
An energetic, high-current-density, broad boron ion beam can be extracted from a pure boron plasma sustained in an ion source by bombarding solid boron with electrons that vaporize it and then ionize the vaporized atoms. Boron ions implanted into a-Fe surfaces held at temperatures above 600 °C for 15 min diffuse over an order of magnitude beyond the ballistic
Acknowledgement This work was supported by the National Science Foundation under Grant No. CMS-9414459.
[1] A.G. von Matuschka, Boronizing, Heyden & Sons, Philadelphia, 1980. [2] F. Miyashita, K. Yokota, Surf. Coat. Technol. 84 (1996) 334. [3] H.J. Kim, W.B. Carter, R.F. Hochman, E.I. Meletis, Mater. Sci. Eng. 69 (1985) 297. [4] N. Millard, G. Marest, N. Moncoffre, J. Tousset, Surf. Coat. Technol. 51 (1992) 446. [5] P.J. Wilbur, R. Wei, Rev. Scient. Instrum. 63 (4) (1992) 2491.
J.A. Davis et al. / Surface and Coatings Technology 103–104 (1998) 52–57 [6 ] ASTM, Annual Book of ASTM Standards, Designation G77-83, v. 03.02, ASTM, Philadelphia, PA, 1983, p. 453. [7] R. Wei, P.J. Wilbur, W.S. Sampath, D.L. Williamson, Y. Qu, L. Wong, J. Tribology 112 (1990) 27–36. [8] P.J. Wilbur, J.A. Davis, D.L. Williamson, J.J. Vajo, R. Wei, Surf. Coat. Technol., 96 (1997) 52–57.
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[9] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Vol. 1, Pergamon, New York, 1985. [10] G.V. Sampsonov, A.P. Epik, in: H.H. Hausner ( Ed.), Coatings of High Temperature Materials, Plenum Press, New York, 1966, p. 2.