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Wear resistance of prenitrided hardcoated steels for tools and machine components
Surfaceand Coatings Technology 88(1996)44-49
Wear resistance of prenitrided hardcoated steels for tools and machine components1 K. H&k a,*, H.-J. Spies a, B. Larisch a, G. Leonhardt b, B. Buecken b a Institutfilr
Werkstofftechnik, Technische Universitlit Bergakademie Freiberg, G. Zeuner StraJe 5, 09596 Freiberg, Germany b Vakuumtechnik Dresden GmbH, BismarckstraJe 66, 01242 Dresden, Germany
Received24 April 1995;acceptedin final form 11February 1996
Abstract Hardenedand temperedlow-alloy steel31CrMoV9 and the high-alloy tool steelsS 6-5-2and X155CrMoV121 werenitrided to form a varied structure of the substratefor the subsequenthardcoating. The tool steelswere nitrided and hardcoated in a continuousprocessin a modified commercialPVD plant. The duplex treatment of the low-alloy steelwas realized by separate nitriding and hardcoating in different plants. The TiN and CrN were depositedwith a thicknessof approx. 3 pm by hollow cathodedischargeevaporation. The composition and structure of the nitrided case,the interstagetreatment before deposition,as well as the deposition parametersinfluencethe propertiesof the composite.The adhesioncan be improved essentiallyby prenitriding and depositionof a gradient interlayer system.The resistanceof the tool steelsto metal cutting and forming increasesdue to the production of an application-specificduplex layer. The resistanceto slidingwear and contact fatigue was investigatedon various duplex-treated low-alloy steelby nitriding the substrate.Whereas the nitrided case has a very high influence on the contact fatigue limit, the hardcoatingreducesthe wear by slidingand abrasion,which is of specialinterestfor machinecomponentswith higher slip.
1. Introduction Casehardening by nitriding has been widely employed industrially, under various circumstances, to improve the wear and corrosion resistance as well as the fatigue limit of constructional parts. It has also been increasingly used for tools in recent years. In comparison with other technologies it is distinguished by an unequalled multiplicity of applications resulting from the nitrided case structure. However, in the case of a very strong surface attack by wear and corrosion, the nitrided casedoes not exhibit sufficient resistance. With an additional protective layer, such as a hardcoating, the load-bearing capacity can be further improved [l]. The sensitivity of nitrided constructional parts towards tribological, chemical and electrochemical attack is mainly controlled by the structure of the compound layer. Their behaviour under cyclic, mechanical and thermal loads depends predominantly on the * Corresponding author. Wingendorfer Strarje BrLunsdorf, Germany. Tel.: 1-49 373214665; fax: f49 ‘Paper presented at the 22nd International Metallurgical Coatings and Thin Films, April 25-29, CA, USA.
78c, D-09603 373214665. Conference on 1995, San Diego,
0257-X972/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved PII SO257-8972(96)02914-3
structure of the precipitation layer [2]. The low fracture toughness of the porous compound. layers is an important disadvantage under dynamic load and for the support of a hardcoating. Nitrided casesof low-alloy steels with depths greater than 0.3 mm and high surface hardness sufficiently support the hardcoating. The high degree of hardness and internal compressive stresses of the case formed by nitriding lead to a high resistance to volume and rolling contact fatigue. The good tempering resistance of the nitrided case allows deposition up to nitriding temperature [2]. The nitrided case of tool steels is much harder than that of low-alloy steels. This leads to an increased resistance to abrasive wear. The very high degree of hardnessand internal compressivestressesin the nitrided case of tool steels reduces the hardness and stress gradient between substrate and a hardcoating stronger than on low-alloy steels. The higher strength of the nitrided case of tool steels is sufficient to support a hardcoating. In previous works, the low-alloy steelswere mostly nitrided with compound layer to study the influence of the sputter cleaning and hardcoating on the composite properties, especially on adhesion. If the
IL H6ck et al. jsurfaee and Coatings Technology 88 (1996) 44-49
I
2. Experimental
Complex stressed machine components and tools /
/
I
2.1. Separate nitviding and hardcoating of low-alloy steels
4
abrasion,
adhesion,
DUPLEX SURFACE TREATMENT I
4.5
,
Fig. 1. Requirements for the surface treatment of complexly stressed parts.
surface temperature exceeds the stability limit of the compound layer, a decomposition of the iron nitride into a soft structure is observed [3,4]; the support of the substrate for the hardcoating is then lost. On the other hand, tool steels with high nitrogen contents (with compound layer) and high diffusion depths lead to a strong embrittlement in the hardened case, which reduces the tool life [5]. The hardcoating, for instance TiN, after nitriding provides a very hard, wear-, heat- and highly chemicalresistant outer layer. Thus, properties obtained by the combination of nitriding and hardcoating allow functional sharing between the core material, the hardened case and the surface, which is of special interest for application in complex stressed parts (Fig. 1). The purpose of the present investigations was to study the influence of the pretreatment of the substrates by plasma and controlled-gas nitriding on the properties of duplex-treated low-alloy steels. Special consideration was given to the production of nitrided case structures, which fulfil the requirements for a good adhesion and wear resistance for both low-alloy steels and high-alloy tool steels.
To attain a relevant increase of the fatigue limit, and rolling contact fatigue limit, the depth of the nitrided case must be approx. 0.4-0.5 mm. For this greater depth of nitrided case it is necessary to nitride the steels for longer times (> 12 h) in both possible processes, plasma and gas nitriding. With regard to these process times, it is advantageous to separate the nitiriding process from the hardcoating process. Hardened and tempered specimens of the nitriding steel 31CrMoV9 (360 HV1) with approx. 2.5% Cr, were ground to a roughness Rz = 0.6-0.8 pm (R, < 0.1 pm). The nitriding of the specimen was performed in industrial plants by controlled-gas nitriding and pulse plasma nitriding to form nitrided cases with nitriding depths greater than 0.4 mm and surface structures both with and without thin compound layers (Table 1). The bright nitriding by gas and plasma nitriding was realized using a two-stage technology: (1) activation of the surface for nitrogen adsorption with higher nitrogen content (nucleation of iron nitrides on the surface); (2) nitriding with a lower nitriding potential or lower nitrogen content in the plasma to inhibit the growth of a compound layer in the outer case. In some cases the nitrided samples were ground or polished before hardcoating to investigate the influence of a mechanical pretreatment before deposition. 2.2. Continuous dqAex treatment for tool steels and deposition conditions An ion plating plant TINA 900 was completed with a pulsed power supply (maximum ON/OFF frequency 33 kHz) [6]. During the plasma nitriding, the samples act as cathodes. The process is started with sputtering in an Ar-H, atmosphere. The nitriding in the cold wall reactor is realized in an N,-H,-Ar atmosphere (Table 2). After pumping the reactor up to high vacuum the hardcoating deposition is carried out. Two different hardened and tempered steel grades were used. The hardened high-speed steel S 6-5-2 (6.4% W, 5.0% MO, 1.8% V, 0.90% C, 4.1% Cr) was tempered to a hardness of 820 HV1. The ledeburitic cold work
Table 1 General process parameters of the nitriding and case properties Nitrided steel
Temp. (“C)
Nitriding time (h)
Case depth (pm)
Hardness HV,
S 6-5-2 plasma X155CrMoV121 plasma 3lCrMoV9 plasma Gas
360-510 360-510 520 550
max 2 max2 30 32
160 150 480 500
<1600 51500 750 780
46
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et al./&uface
and Coatings
Technology
88 (1996)
44-49
Table 2 Parameters of the continuous plasma nitriding and TiN hardcoating and separate hardcoating Substrate temp. (“C)
Nitriding” (120 min) TiNb (40 min)
360-510 350
Voltage (V)
250-420 38
Effective current (A)
4.1-5.3 220
Pressure (Pa)
Gas mixture (%)
300 0.25
Pulse on/off (a)
Ar
N,
H,
20 60
40 40
40 -
50/50 15160
“Pulsed dc discharge. bHollow cathode discharge evaporation.
steel XlSSCrMoV121 (1.55% C, 12% Cr, 1.0% MO, 1.2% V) was treated to a hardness of 630 HV1. TiN or CrN hardcoatings were deposited by a reactive hollow cathode discharge evaporation, using the pulsed power supply for substrate bias. Some samples were moved by a planet rotating system around the evaporation anode to realize a 3-dimensional hardcoating. The TiN and CrN was deposited at 350°C with a coating thickness of approx. 3 ym (Table 2). After separate nitriding and hardcoating a sputter cleaning by argon ion bombardment was carried out before deposition to remove impurities.
HighspeedsteelS 6-5-2 500
0
05
1
I,5
Nitrogen
3. Results and discussion 3.1. Properties of continuousduplex-treated tool steels
The growth, structure and properties of nitrided layers are influenced by the nitriding temperature, the gas composition and the alloying elements of the steel. The high chromium content in the tool steels results in a high nitrogen content in the nitrided case without forming a compound layer. With increasing nitriding temperature the hardnesses of the ledeburitic cold-work steel and the high-speed steel increase up to 1400 and 1600 HVo,I, respectively. For cutting and forming tool steels it is necessary to minimize the embrittlement of the outer case resulting from the nitriding. Therefore, the nitriding depth should not higher than 50 pm. The hardness of the TiN on the surface is influenced by the hardness of the nitrided substrate and by the hardcoating structure (Fig. 2). With increasing substrate hardness, resulting from a higher nitrogen content in the substrate, the measured hardness increases. But on substrates nitrided at higher temperatures this measured compound hardness decreases. Because of the higher surface roughness of the nitrided steel with increasing nitriding temperature, the structure of the TiN becomes less dense. The most important effect of the continuous combination of nitriding and hardcoating is the increase in the adhesion. Fig. 3 shows results of scratch tests: a Rockwell C diamond scratched with continuous increasing load, up to 200 N, over the hardcoated surface. Criteria for
Content
2
2,5
in the Substrate
3
3,s
4
0
[Wt.-%]
Fig. 2. Composite hardness and microhardness of TiN in relation to the maximum nitrogen content in the nitrided surface of the steel S 6-5-2.
“not
nitrided
360°C Nitriding
410°C
460°C
510°C
-’
Temperature
Fig. 3. Critical loads of the !?irstadhesive failure of the TiN on various nitrided tool steel: (a) S 6-5-2; (b) X155CrMoV121.
adhesion are the loads at which the hardcoating delaminates in or around the scratch trace. With increasing nitriding temperature, the critical loads for the TiNcoated tool steels increases strongly from 60 to 180 N, as shown in Fig. 3. The deformation of the hardcoating and the resulting stresses on the interface can be reduced by hardening the substrate. The internal stress gradient between hardcoating and substrate decreases also. For example, in the nitrided layer of high-speed steel, compressive residual stresses up to 1100 MPa were measured. To characterize the wear resistance in metal contact
K H&k
et aI./Surface
and Coatings
- important
for tools - the duplex-treated steels were tested in a special sliding wear test. A disk (0.7 mm thickness) of a structural steel with 0.45% carbon rotates with a constant load towards the test piece until the layer is removed. The wear rate was determined as a function of the trace depth and the sliding distance. The increase of the wear resistance of the duplex-treated steels shows the same dependence on the nitriding conditions as the scratch test (Fig. 4). The hardcoating reduces the sliding wear coefficient. Furthermore, the higher adhesion of the hardcoating to an optimal prenitrided substrate reduces ,the wear essentially. The tool behaviour for metal cutting and forming can be significantly improved.
Technology
88 (1996)
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41
Table 3 Critical loads of the first adhesive failure of the coating on various pretreated nitrided 31CrMoV9 Layer structure
Critical load (N)
Plasma nitrided with compound layer and TiN Plasma nitrided, polished and TiN Plasma bright nitrided and TiN Plasma bright nitrided, polished and TiN Gas nitrided with compound layer and TiN Gas uitrided, polished and TiN Gas nitrided, compound layer removed and TiN Gas bright nitrided and TiN Gas bright nitrided, polished and TiN Nitrided, Ti-TiN, gradient interlayer and TiN bright nittided, Ti-TiN, gradient interlayer and TiN
31 35 67 42 0 29 43 65 53 67 77
3.2. Duplex surface engineeringof low-alloy steels
Nitriding of low-alloy steels produces nuclei of iron nitrides after the saturation of E-iron with nitrogen. These iron nitrides preferentially grow in a lateral direction to form a top layer [4]. Recent investigations of the adhesion of this top layer with tests on gas nitrided surfaces by scratch have shown an extensive flaking-off of this cover layer from the original surface or the compound layer. The nitride top layer on plasma nitrided steels with a compound layer is not as strongly developed and has a higher adhesion than on gas nitrided samples. Scratch tests on TiN-deposited bright nitrided 3 lCrMoV9 without mechanical pretreatment show higher critical loads for adhesive failure than on polished surfaces (Table 3). This may be an effect of the morphological influence of the iron nitride cover layer on TiN growth. If the compound layer is ground upon adhesion of TiN, the highest critical loads in the scratch test are obtained on bright, gas and plasma nitrided steels. An essential improvement of adhesion, more than for
Fig. 4. Sliding wear of continuous duplex-treated tool steels in a plane on disk test: (a) S 6-5-2; (b) X155CrMoV121.
a single Ti intermediate layer, can be attained by a gradient Ti-TiN, interlayer (Table 3). The deposition of this Ti-TiN, intermediate layer reduces the hardness gradient and residual stress gradient between the substrate and the stoichiometric TiN. The sliding wear behaviour of various treated samples was characterized in a block-on-roll test. The blocks were tested under dry sliding conditions in linear contact on a rotating roll of hardened and tempered X155CrR;IoV121. Fig. 5 shows the decrease of the sliding wear by deposition of TiN on the nitrided steel in comparison to only nitrided steels. The higher linear wear of the nitrided 31CrMoV9 with a compound layer may be caused by the brittle porous outer part of the compound layer (wear depth <2 pm). Best results with regard to the resistance to sliding contact was attained for the nitrided and CrN-coated samples. The wear on non-coated surfaces was increased by the formation of iron oxide, which was detected by
Fig. 5. Sliding wear of nitrided and duplex-treated 31CrMoV9 plane-on-roll test.
in the
48
K H&k
et al./Surface
and Coatings
X-ray diffraction. This tribochemical reaction takes place between test piece and roll, and results in an increase of wear due to the abrasive effect of the iron oxide. This was not observed on hardcoated surfaces, except when the hardcoating was locally removed in the wear trace. The influence of hardcoating on the fatigue limit under rolling contact, tested in a double disk machine, is obviously low (Fig. 6). The maximum stress under rolling contact occurs in the subsurface. The thin surface layer has no essential influence on the fatigue limit. The high fatigue limit of nitrided steels is a result of the increased hardness and the formation of compressive residual stresses in the hardened case. Pittings, as the indication for test piece failure, begins in the subsurface. The bright nitrided 31CrMoV9 has a lower fatigue limit than the nitrided steel with a compound layer. Whereas bright nitriding leads to a higher maximum of compressive residual stress in the subsurface (580 MPa) than the nitriding with a compound layer (450 MPa), the maximum hardness of the bright nitrided case and the hardening depth is lower. The rolling contact fatigue limit decreases, with a small difference of the absolute values. The thin hardcoating is only able to reduce the adhesive and abrasive wear on the surface on metal contact, In contrast to the wear behaviour, the hardcoating of the compound layer is advantageous for use in corrosive media (Fig. 7). Whereas the matrix material and the TiN-coated steel dissolved rapidly under anodic polarisation, the nitrided steel surfaces are able to passivate in NaCl solution. This leads to a better corrosion resistance, because normally the TiN coating has defects and pores. The passive current density of the TiN-coated nitrided steel is one order of magnitude lower than that of the nitrided only steel. The electrochemical potential differ-
Technology
88 (1996)
44-49
ence between TiN and iron nitride is lower than between TiN and iron. The pitting potential of the TiN-coated nitrided steel is about 250 mV more noble in comparison to the nitrided only steel. These results show that the combination of an iron nitride underlayer and a TiN top layer results in a significant improvement of the corrosion behaviour of duplex-treated low-alloy steels.
4. Conclusions Two methods of duplex surface engineering for various applications, which differ in process technologies and the resulting improvements, are shown: 4.1. The continuous combination of short-time nitriding and hardcoating for high-alloy steelsfor higher wear resistanceof cutting andforming tools
This leads to a material composition with an higher adhesion of hardcoating. As a result of the optimized technology, the protection afforded to cutting and forming tools against adhesive and abrasive wear can be improved. The continuous vacuum process of nitriding and hardcoating without external steps prior to coating results in improved properties of tool steels and has subsequent economical advantages concerning the purchase of the equipment as well as the costs of energy, gas and the cooling of the duplex treatment. 4.2. The separatenitriding and hardcoating of low-alloy steelsfor highly resistant machine components
A composite of a very hard outer layer, a thicker hardened case and a tough core is of special interest, if abrasive or sliding wear and rolling contact or volume fatigue occur together. Plasma nitriding with dense nitride cover layers, bright nitriding or removal of such coarse surface structures results in a higher adhesion of the hardcoating. The adhesion can be further improved by deposition of a gradient Ti-TiN, intermediate layer. Hardcoating of nitrided steel reduces the friction and sliding wear substantially. Tribochemical reactions are reduced. The fatigue limit in rolling contact increases with nitriding. Thin coating has a small influence on the wear behaviour under rolling contact. Corrosive wear can be reduced by producing a iron nitride compound layer before hardcoating. Acknowledgement
Fig. 6. Results of testing rolling-butt contact of nitrided and duplextreated 31CrMoV9 in a double disk machine (lubricated).
The authors are indebted to the Deutsche Forschungs Gemeinschaft (Pr.-No. Sp 376/7-l), the Ministry of Economy of Saxonia (Pr.-No. 8/2) and the Stiftung
K H&k et al. JSurface and Coatings Technology 88 (1996)
Solution: %
-1
-03
o
0
49
44-49
0.9 M (5%) NaCl ,
-a’-L-&-
03
1
1,s
2
potential u w,scq Fig. 7. Potential-current
density function of nitrided, hardcoated and duplex-treated low-alloy steel tested in 0.9 M NaCl solution.
Stahlanwendung e.V (PL-No. P 234/14/92/S24/4/92) the financial support of the investigations.
for
References [ 11 Y. Sun and T. Bell, iMater+. Sci. Eng., A140 (1991) 419-434. [2] H.-J. Spies, H.-P.Winkler and 3. Langenhahn, H&her&Techn. Mitt.. 44 (1989) 75-82.
[3] M. Zlatanovic and T. Gredic, Mater. Sci. Fo’orzm,102-104 (1992) Switzerland, 655-666. r4] H.-J. Spies, K. Hock, E. Broszeit, W. Herr and B. Matthes, Surf. Coat. Technol., 60 (1993) 443. [5] W. Hoffmann, PhD, TU Bergakademie Freiberg (Germany), 1987. [6] B. Buecken, G. Leonhardt, R. Wilberg, K. Hoeck and H.-J. Spies, Surf. Coat. Technol., 68/69 (1994) 244-248.
Wear resistance of prenitrided hardcoated steels for tools and machine components1 K. H&k a,*, H.-J. Spies a, B. Larisch a, G. Leonhardt b, B. Buecken b a Institutfilr
Werkstofftechnik, Technische Universitlit Bergakademie Freiberg, G. Zeuner StraJe 5, 09596 Freiberg, Germany b Vakuumtechnik Dresden GmbH, BismarckstraJe 66, 01242 Dresden, Germany
Received24 April 1995;acceptedin final form 11February 1996
Abstract Hardenedand temperedlow-alloy steel31CrMoV9 and the high-alloy tool steelsS 6-5-2and X155CrMoV121 werenitrided to form a varied structure of the substratefor the subsequenthardcoating. The tool steelswere nitrided and hardcoated in a continuousprocessin a modified commercialPVD plant. The duplex treatment of the low-alloy steelwas realized by separate nitriding and hardcoating in different plants. The TiN and CrN were depositedwith a thicknessof approx. 3 pm by hollow cathodedischargeevaporation. The composition and structure of the nitrided case,the interstagetreatment before deposition,as well as the deposition parametersinfluencethe propertiesof the composite.The adhesioncan be improved essentiallyby prenitriding and depositionof a gradient interlayer system.The resistanceof the tool steelsto metal cutting and forming increasesdue to the production of an application-specificduplex layer. The resistanceto slidingwear and contact fatigue was investigatedon various duplex-treated low-alloy steelby nitriding the substrate.Whereas the nitrided case has a very high influence on the contact fatigue limit, the hardcoatingreducesthe wear by slidingand abrasion,which is of specialinterestfor machinecomponentswith higher slip.
1. Introduction Casehardening by nitriding has been widely employed industrially, under various circumstances, to improve the wear and corrosion resistance as well as the fatigue limit of constructional parts. It has also been increasingly used for tools in recent years. In comparison with other technologies it is distinguished by an unequalled multiplicity of applications resulting from the nitrided case structure. However, in the case of a very strong surface attack by wear and corrosion, the nitrided casedoes not exhibit sufficient resistance. With an additional protective layer, such as a hardcoating, the load-bearing capacity can be further improved [l]. The sensitivity of nitrided constructional parts towards tribological, chemical and electrochemical attack is mainly controlled by the structure of the compound layer. Their behaviour under cyclic, mechanical and thermal loads depends predominantly on the * Corresponding author. Wingendorfer Strarje BrLunsdorf, Germany. Tel.: 1-49 373214665; fax: f49 ‘Paper presented at the 22nd International Metallurgical Coatings and Thin Films, April 25-29, CA, USA.
78c, D-09603 373214665. Conference on 1995, San Diego,
0257-X972/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved PII SO257-8972(96)02914-3
structure of the precipitation layer [2]. The low fracture toughness of the porous compound. layers is an important disadvantage under dynamic load and for the support of a hardcoating. Nitrided casesof low-alloy steels with depths greater than 0.3 mm and high surface hardness sufficiently support the hardcoating. The high degree of hardness and internal compressive stresses of the case formed by nitriding lead to a high resistance to volume and rolling contact fatigue. The good tempering resistance of the nitrided case allows deposition up to nitriding temperature [2]. The nitrided case of tool steels is much harder than that of low-alloy steels. This leads to an increased resistance to abrasive wear. The very high degree of hardnessand internal compressivestressesin the nitrided case of tool steels reduces the hardness and stress gradient between substrate and a hardcoating stronger than on low-alloy steels. The higher strength of the nitrided case of tool steels is sufficient to support a hardcoating. In previous works, the low-alloy steelswere mostly nitrided with compound layer to study the influence of the sputter cleaning and hardcoating on the composite properties, especially on adhesion. If the
IL H6ck et al. jsurfaee and Coatings Technology 88 (1996) 44-49
I
2. Experimental
Complex stressed machine components and tools /
/
I
2.1. Separate nitviding and hardcoating of low-alloy steels
4
abrasion,
adhesion,
DUPLEX SURFACE TREATMENT I
4.5
,
Fig. 1. Requirements for the surface treatment of complexly stressed parts.
surface temperature exceeds the stability limit of the compound layer, a decomposition of the iron nitride into a soft structure is observed [3,4]; the support of the substrate for the hardcoating is then lost. On the other hand, tool steels with high nitrogen contents (with compound layer) and high diffusion depths lead to a strong embrittlement in the hardened case, which reduces the tool life [5]. The hardcoating, for instance TiN, after nitriding provides a very hard, wear-, heat- and highly chemicalresistant outer layer. Thus, properties obtained by the combination of nitriding and hardcoating allow functional sharing between the core material, the hardened case and the surface, which is of special interest for application in complex stressed parts (Fig. 1). The purpose of the present investigations was to study the influence of the pretreatment of the substrates by plasma and controlled-gas nitriding on the properties of duplex-treated low-alloy steels. Special consideration was given to the production of nitrided case structures, which fulfil the requirements for a good adhesion and wear resistance for both low-alloy steels and high-alloy tool steels.
To attain a relevant increase of the fatigue limit, and rolling contact fatigue limit, the depth of the nitrided case must be approx. 0.4-0.5 mm. For this greater depth of nitrided case it is necessary to nitride the steels for longer times (> 12 h) in both possible processes, plasma and gas nitriding. With regard to these process times, it is advantageous to separate the nitiriding process from the hardcoating process. Hardened and tempered specimens of the nitriding steel 31CrMoV9 (360 HV1) with approx. 2.5% Cr, were ground to a roughness Rz = 0.6-0.8 pm (R, < 0.1 pm). The nitriding of the specimen was performed in industrial plants by controlled-gas nitriding and pulse plasma nitriding to form nitrided cases with nitriding depths greater than 0.4 mm and surface structures both with and without thin compound layers (Table 1). The bright nitriding by gas and plasma nitriding was realized using a two-stage technology: (1) activation of the surface for nitrogen adsorption with higher nitrogen content (nucleation of iron nitrides on the surface); (2) nitriding with a lower nitriding potential or lower nitrogen content in the plasma to inhibit the growth of a compound layer in the outer case. In some cases the nitrided samples were ground or polished before hardcoating to investigate the influence of a mechanical pretreatment before deposition. 2.2. Continuous dqAex treatment for tool steels and deposition conditions An ion plating plant TINA 900 was completed with a pulsed power supply (maximum ON/OFF frequency 33 kHz) [6]. During the plasma nitriding, the samples act as cathodes. The process is started with sputtering in an Ar-H, atmosphere. The nitriding in the cold wall reactor is realized in an N,-H,-Ar atmosphere (Table 2). After pumping the reactor up to high vacuum the hardcoating deposition is carried out. Two different hardened and tempered steel grades were used. The hardened high-speed steel S 6-5-2 (6.4% W, 5.0% MO, 1.8% V, 0.90% C, 4.1% Cr) was tempered to a hardness of 820 HV1. The ledeburitic cold work
Table 1 General process parameters of the nitriding and case properties Nitrided steel
Temp. (“C)
Nitriding time (h)
Case depth (pm)
Hardness HV,
S 6-5-2 plasma X155CrMoV121 plasma 3lCrMoV9 plasma Gas
360-510 360-510 520 550
max 2 max2 30 32
160 150 480 500
<1600 51500 750 780
46
K H&k
et al./&uface
and Coatings
Technology
88 (1996)
44-49
Table 2 Parameters of the continuous plasma nitriding and TiN hardcoating and separate hardcoating Substrate temp. (“C)
Nitriding” (120 min) TiNb (40 min)
360-510 350
Voltage (V)
250-420 38
Effective current (A)
4.1-5.3 220
Pressure (Pa)
Gas mixture (%)
300 0.25
Pulse on/off (a)
Ar
N,
H,
20 60
40 40
40 -
50/50 15160
“Pulsed dc discharge. bHollow cathode discharge evaporation.
steel XlSSCrMoV121 (1.55% C, 12% Cr, 1.0% MO, 1.2% V) was treated to a hardness of 630 HV1. TiN or CrN hardcoatings were deposited by a reactive hollow cathode discharge evaporation, using the pulsed power supply for substrate bias. Some samples were moved by a planet rotating system around the evaporation anode to realize a 3-dimensional hardcoating. The TiN and CrN was deposited at 350°C with a coating thickness of approx. 3 ym (Table 2). After separate nitriding and hardcoating a sputter cleaning by argon ion bombardment was carried out before deposition to remove impurities.
HighspeedsteelS 6-5-2 500
0
05
1
I,5
Nitrogen
3. Results and discussion 3.1. Properties of continuousduplex-treated tool steels
The growth, structure and properties of nitrided layers are influenced by the nitriding temperature, the gas composition and the alloying elements of the steel. The high chromium content in the tool steels results in a high nitrogen content in the nitrided case without forming a compound layer. With increasing nitriding temperature the hardnesses of the ledeburitic cold-work steel and the high-speed steel increase up to 1400 and 1600 HVo,I, respectively. For cutting and forming tool steels it is necessary to minimize the embrittlement of the outer case resulting from the nitriding. Therefore, the nitriding depth should not higher than 50 pm. The hardness of the TiN on the surface is influenced by the hardness of the nitrided substrate and by the hardcoating structure (Fig. 2). With increasing substrate hardness, resulting from a higher nitrogen content in the substrate, the measured hardness increases. But on substrates nitrided at higher temperatures this measured compound hardness decreases. Because of the higher surface roughness of the nitrided steel with increasing nitriding temperature, the structure of the TiN becomes less dense. The most important effect of the continuous combination of nitriding and hardcoating is the increase in the adhesion. Fig. 3 shows results of scratch tests: a Rockwell C diamond scratched with continuous increasing load, up to 200 N, over the hardcoated surface. Criteria for
Content
2
2,5
in the Substrate
3
3,s
4
0
[Wt.-%]
Fig. 2. Composite hardness and microhardness of TiN in relation to the maximum nitrogen content in the nitrided surface of the steel S 6-5-2.
“not
nitrided
360°C Nitriding
410°C
460°C
510°C
-’
Temperature
Fig. 3. Critical loads of the !?irstadhesive failure of the TiN on various nitrided tool steel: (a) S 6-5-2; (b) X155CrMoV121.
adhesion are the loads at which the hardcoating delaminates in or around the scratch trace. With increasing nitriding temperature, the critical loads for the TiNcoated tool steels increases strongly from 60 to 180 N, as shown in Fig. 3. The deformation of the hardcoating and the resulting stresses on the interface can be reduced by hardening the substrate. The internal stress gradient between hardcoating and substrate decreases also. For example, in the nitrided layer of high-speed steel, compressive residual stresses up to 1100 MPa were measured. To characterize the wear resistance in metal contact
K H&k
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for tools - the duplex-treated steels were tested in a special sliding wear test. A disk (0.7 mm thickness) of a structural steel with 0.45% carbon rotates with a constant load towards the test piece until the layer is removed. The wear rate was determined as a function of the trace depth and the sliding distance. The increase of the wear resistance of the duplex-treated steels shows the same dependence on the nitriding conditions as the scratch test (Fig. 4). The hardcoating reduces the sliding wear coefficient. Furthermore, the higher adhesion of the hardcoating to an optimal prenitrided substrate reduces ,the wear essentially. The tool behaviour for metal cutting and forming can be significantly improved.
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Table 3 Critical loads of the first adhesive failure of the coating on various pretreated nitrided 31CrMoV9 Layer structure
Critical load (N)
Plasma nitrided with compound layer and TiN Plasma nitrided, polished and TiN Plasma bright nitrided and TiN Plasma bright nitrided, polished and TiN Gas nitrided with compound layer and TiN Gas uitrided, polished and TiN Gas nitrided, compound layer removed and TiN Gas bright nitrided and TiN Gas bright nitrided, polished and TiN Nitrided, Ti-TiN, gradient interlayer and TiN bright nittided, Ti-TiN, gradient interlayer and TiN
31 35 67 42 0 29 43 65 53 67 77
3.2. Duplex surface engineeringof low-alloy steels
Nitriding of low-alloy steels produces nuclei of iron nitrides after the saturation of E-iron with nitrogen. These iron nitrides preferentially grow in a lateral direction to form a top layer [4]. Recent investigations of the adhesion of this top layer with tests on gas nitrided surfaces by scratch have shown an extensive flaking-off of this cover layer from the original surface or the compound layer. The nitride top layer on plasma nitrided steels with a compound layer is not as strongly developed and has a higher adhesion than on gas nitrided samples. Scratch tests on TiN-deposited bright nitrided 3 lCrMoV9 without mechanical pretreatment show higher critical loads for adhesive failure than on polished surfaces (Table 3). This may be an effect of the morphological influence of the iron nitride cover layer on TiN growth. If the compound layer is ground upon adhesion of TiN, the highest critical loads in the scratch test are obtained on bright, gas and plasma nitrided steels. An essential improvement of adhesion, more than for
Fig. 4. Sliding wear of continuous duplex-treated tool steels in a plane on disk test: (a) S 6-5-2; (b) X155CrMoV121.
a single Ti intermediate layer, can be attained by a gradient Ti-TiN, interlayer (Table 3). The deposition of this Ti-TiN, intermediate layer reduces the hardness gradient and residual stress gradient between the substrate and the stoichiometric TiN. The sliding wear behaviour of various treated samples was characterized in a block-on-roll test. The blocks were tested under dry sliding conditions in linear contact on a rotating roll of hardened and tempered X155CrR;IoV121. Fig. 5 shows the decrease of the sliding wear by deposition of TiN on the nitrided steel in comparison to only nitrided steels. The higher linear wear of the nitrided 31CrMoV9 with a compound layer may be caused by the brittle porous outer part of the compound layer (wear depth <2 pm). Best results with regard to the resistance to sliding contact was attained for the nitrided and CrN-coated samples. The wear on non-coated surfaces was increased by the formation of iron oxide, which was detected by
Fig. 5. Sliding wear of nitrided and duplex-treated 31CrMoV9 plane-on-roll test.
in the
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X-ray diffraction. This tribochemical reaction takes place between test piece and roll, and results in an increase of wear due to the abrasive effect of the iron oxide. This was not observed on hardcoated surfaces, except when the hardcoating was locally removed in the wear trace. The influence of hardcoating on the fatigue limit under rolling contact, tested in a double disk machine, is obviously low (Fig. 6). The maximum stress under rolling contact occurs in the subsurface. The thin surface layer has no essential influence on the fatigue limit. The high fatigue limit of nitrided steels is a result of the increased hardness and the formation of compressive residual stresses in the hardened case. Pittings, as the indication for test piece failure, begins in the subsurface. The bright nitrided 31CrMoV9 has a lower fatigue limit than the nitrided steel with a compound layer. Whereas bright nitriding leads to a higher maximum of compressive residual stress in the subsurface (580 MPa) than the nitriding with a compound layer (450 MPa), the maximum hardness of the bright nitrided case and the hardening depth is lower. The rolling contact fatigue limit decreases, with a small difference of the absolute values. The thin hardcoating is only able to reduce the adhesive and abrasive wear on the surface on metal contact, In contrast to the wear behaviour, the hardcoating of the compound layer is advantageous for use in corrosive media (Fig. 7). Whereas the matrix material and the TiN-coated steel dissolved rapidly under anodic polarisation, the nitrided steel surfaces are able to passivate in NaCl solution. This leads to a better corrosion resistance, because normally the TiN coating has defects and pores. The passive current density of the TiN-coated nitrided steel is one order of magnitude lower than that of the nitrided only steel. The electrochemical potential differ-
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ence between TiN and iron nitride is lower than between TiN and iron. The pitting potential of the TiN-coated nitrided steel is about 250 mV more noble in comparison to the nitrided only steel. These results show that the combination of an iron nitride underlayer and a TiN top layer results in a significant improvement of the corrosion behaviour of duplex-treated low-alloy steels.
4. Conclusions Two methods of duplex surface engineering for various applications, which differ in process technologies and the resulting improvements, are shown: 4.1. The continuous combination of short-time nitriding and hardcoating for high-alloy steelsfor higher wear resistanceof cutting andforming tools
This leads to a material composition with an higher adhesion of hardcoating. As a result of the optimized technology, the protection afforded to cutting and forming tools against adhesive and abrasive wear can be improved. The continuous vacuum process of nitriding and hardcoating without external steps prior to coating results in improved properties of tool steels and has subsequent economical advantages concerning the purchase of the equipment as well as the costs of energy, gas and the cooling of the duplex treatment. 4.2. The separatenitriding and hardcoating of low-alloy steelsfor highly resistant machine components
A composite of a very hard outer layer, a thicker hardened case and a tough core is of special interest, if abrasive or sliding wear and rolling contact or volume fatigue occur together. Plasma nitriding with dense nitride cover layers, bright nitriding or removal of such coarse surface structures results in a higher adhesion of the hardcoating. The adhesion can be further improved by deposition of a gradient Ti-TiN, intermediate layer. Hardcoating of nitrided steel reduces the friction and sliding wear substantially. Tribochemical reactions are reduced. The fatigue limit in rolling contact increases with nitriding. Thin coating has a small influence on the wear behaviour under rolling contact. Corrosive wear can be reduced by producing a iron nitride compound layer before hardcoating. Acknowledgement
Fig. 6. Results of testing rolling-butt contact of nitrided and duplextreated 31CrMoV9 in a double disk machine (lubricated).
The authors are indebted to the Deutsche Forschungs Gemeinschaft (Pr.-No. Sp 376/7-l), the Ministry of Economy of Saxonia (Pr.-No. 8/2) and the Stiftung
K H&k et al. JSurface and Coatings Technology 88 (1996)
Solution: %
-1
-03
o
0
49
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0.9 M (5%) NaCl ,
-a’-L-&-
03
1
1,s
2
potential u w,scq Fig. 7. Potential-current
density function of nitrided, hardcoated and duplex-treated low-alloy steel tested in 0.9 M NaCl solution.
Stahlanwendung e.V (PL-No. P 234/14/92/S24/4/92) the financial support of the investigations.
for
References [ 11 Y. Sun and T. Bell, iMater+. Sci. Eng., A140 (1991) 419-434. [2] H.-J. Spies, H.-P.Winkler and 3. Langenhahn, H&her&Techn. Mitt.. 44 (1989) 75-82.
[3] M. Zlatanovic and T. Gredic, Mater. Sci. Fo’orzm,102-104 (1992) Switzerland, 655-666. r4] H.-J. Spies, K. Hock, E. Broszeit, W. Herr and B. Matthes, Surf. Coat. Technol., 60 (1993) 443. [5] W. Hoffmann, PhD, TU Bergakademie Freiberg (Germany), 1987. [6] B. Buecken, G. Leonhardt, R. Wilberg, K. Hoeck and H.-J. Spies, Surf. Coat. Technol., 68/69 (1994) 244-248.