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Properties of surface layers on titanium alloy produced by thermo-chemical treatments under glow discharge conditions
6/MJ'/-E
CMTINGS ,
ELSEVIER
Surface and Coatings Technology96 (1997) 205-209
HtllNOID6Y
Properties of surface layers on titanium alloy produced by thermo-chemical treatments under glow discharge conditions T. W i e r z c h o f i *, A . F l e s z a r
Warsaw University of Technology, Faculty of Materials Science and Engineering, ul. Narbutta 85, 02-524, Warszawa, Poland Received 20 January 1997; accepted 21 March 1997
Abstract The recent rapid progress in surface treatment techniques dictates that the titanium alloys should have an improved resistance to frictional wear without any loss of their high corrosion resistance. These requirements can be satisfied by producing surface layers of specified microstructure and phase composition. The present paper describes a modification of the plasma discharge nitriding treatment of titanium alloys, i.e. the glow discharge-assisted oxycarbonitriding, which by introducing oxygen, nitrogen and carbon into the surface zone of the layer [a Ti(NCO) type layer] improves its useful properties, primarily the resistance to frictional wear and the resistance to corrosion [1,2]. This is because titanium shows a good affinity to oxygen, carbon and nitrogen, whereas the chemical composition of the layer depends on the chemical composition of the low-temperature plasma that forms under the conditions of glow discharge. The present paper also discusses the results of the corrosion behaviour of the OT4-0 titanium alloy in simulated human body fluids before and after the sterilization process. The isothermal and cyclic plasma nitriding and carboxynitriding processes on titanium alloys can be carried out in a specially modified glow discharge chamber designed for the plasma nitriding process. © i997 Elsevier Science S.A.
Ke)words: Friction; Titanium; Corrosion; Glow discharge chamber
1. Introduction
2. Experimental procedure
That titanium and its alloys are widely used today is associated with the specific physical and chemical properties of this metal. The range of its application is still being widened as the methods of its production are improved so as to control better its properties. Titanium is an attractive structural material, possessing a small specific weight, a very good resistance to corrosion and a high relative strength, but unfortunately, also a low resistance to frictional wear due, among other things, to the formation of adhesive joints and the relatively high friction coefficient. For this reason, various methods of surface treatment are used, such as, for example, glow discharge nitriding, thermal spraying, PVD treatments, anodic oxidation, electroless chemical deposition of metals, ion implantation or laser treatment [3-6]. These techniques are being increasingly used since essentially they improve the properties of products made of titanium and its alloys. Titanium and titanium alloys are widely used today in dentistry and orthopedic surgery because of their good biocompatibility.
Specimens of the OT4-0 titanium alloy (0.4-1.4%A1, 0.5-1.3%Mn, 0.3%Fe, 0.3%Cr, 0.12%Si) were subjected to plasma nitriding and oxycarbonitriding at a temperature of 850 °C for 4 h (isothermal processes) and cyclically between 730 and 930°C for 4 h (between the c ~ + / ~ p transformation temperature). The specimens were subjected to plasma treatment in the gas atmosphere composed of N2 (nitriding) and the vapours of organic compounds +nitrogen (oxycarbonitriding process) at a pressure of about 5 hPa. The layers thus obtained were examined metallographically. The study also included examinations of the phase composition using a Philips 1830 type X-ray difractometer, and wear and corrosion resistance tests. The tribological characteristics of the surface after the treatment were evaluated using the ~three roller-taper' method [7,8]. In this test, friction is applied, under specified conditions, between three fixed cylindrical specimens (rollers) 8 mm in diameter and a rotating conical counter specimen (taper). The linear wear, expressed as the wear depth, was determined by measuring the diameters of the ellipses formed on
* Corresponding author.
0257-8972/97/$17.00 © I997 ElsevierScienceS.A. All rights reserved. PHS0257-8972(97)00113-8
206
Z Wierzchoh, A. Fleszar / Surface and Coatings Technology 96 (1997) 205-209
the surfaces of each roller. The results were then averaged. The counter specimen was made of AISI-45 steel quench hardened and tempered to a hardness of 30 HRC. A constant surface pressure of 100 and 200 MPa was applied. The corrosion resistance test was conducted using the potentiodynamic method in a 1.8 M H2S0 4 solution. The reference electrode was a saturated calomel electrode. Anodic polarization curves were performed at a temperature of 25 °C using a Taccusel PRT-20 potentiostat. Potentiodynamic polarization of the OT4-0 alloy before and after the sterilization process was performed in a special solution (containing inorganic salts, amino acids, vitamins and antibiotics--for example penicillin and streptomycin) using the potentiostat. The sterilization process was conducted in steam at 141 °C and a pressure of 1500 hPa. A temperature of 37 °C, the human body temperature, was maintained during the entire experiment. A saturated calomel electrode (SCE) was used as the reference and a platinium electrode as the counter. The microhardnesses of the polished surfaces of specimen cross-sections were measured using a Vickers indenter under a 50 G load. The specimens were etched using a mixture of HF and HNO3 acids. The etched cross-sections of the specimens were observed using a Nephot-2 light microscope. The surfaces of the layers were observed using a Tesla scanning electron microscope of the BT300 type.
3. Results and discussion
Fig. 1 shows the microstructures of the surface layers produced on the OT4-0 alloy by isothermal and cyclic plasma nitriding and oxycarbonitriding processes. The surface hardnesses of these layers range from 1800 to 1900 HV0.05 after cyclic nitriding and oxycarbonitriding and from 2200 to 2400 HV0.05 after isothermal nitriding and oxycarbonitriding. The nitriding processes produce TiN +Ti2N + c~Ti(N) (a)
(b)
type layers and oxycarbonitriding Ti(NCO)+TizN + c~Ti(N) type layers. The thickness of TiN or Ti (NCO) was about 2 - 4 g m and that of Ti2N was about 35-40 ~m. Fig. 2 compares the measured values of the corrosion resistance of the Ti(NCO)+TizN+c~Ti(N) and TiN+Ti2N+c~Ti(N) type layers produced by isothermal and cyclic plasma nitriding and oxycarbonitriding treatments of the titanium alloy OT4-0 with those obtained for the OT4-0 alloy alone in t.8 M }-I2S04. AS follows from Fig. 2, the best corrosion resistance is shown by the OT4-0 alloy subjected to cyclic temperature glow discharge oxycarbonitriding. This is because the titanium oxycarbonitride layer thus obtained shows the best regularity and homogenity (Fig. 1). The results obtained by scanning electron microscopy (Fig. 3) have shown that the topography of the surface layers varies depending on the process conditions, and thus also depending on the carbon, nitrogen and oxygen contents in the layer. This in turn considerably affects the properties of the surface layers, in particular their resistance to corrosion and frictional wear. Fig. 4 compares the results obtained from examinations of the resistance to frictional wear of the TiN+Ti2N+~Ti(N) and Ti(NCO)+TizN+~Ti(N ) layers produced on the OT4-0 alloy with those obtained for the OT4-0 alone. We can also see that the layers obtained by plasma processes on the OT4-0 titanium alloy have in addition a good resistance to frictional wear. At a load of 100MPa the specimens of the OT4-0 titanium alloy underwent seizure after 10 min. Moreover, the corrosion resistance of the OT4-0 alloy subjected to isothermal and cyclic nitriding has not changed after the sterilization process (see Fig. 5).
4. Conclusions
The plasma nitriding and carboxynitriding techniques developed in the present experiments increases the resis(c)
(d)
Fig. 1. Microstructures of the surfacelayersproduced on the OT4-0 alloy by isothermal plasma nitriding (a) and oxycarbonitriding(c) and cyclic plasma nitriding (b) and oxycarbonitridingprocesses(d).
207
T. Wierzchofi, A. Fleszar / Surface and Coatings Technology 96 (1997) 205-209 10000
OT4-Oafter isothermaloxycarbonitriding . . . . . . . . . . . . . . . . . . . . . . . 10o0 , . ° . . .~. . .
100
,
,"
\
.'/
f
~ after i isothe n g
I,.\.I/-.
oT4-oaone
/ %--
~ "'4'~
//:
?" .--"
..':" .I.."
gO "O
\
\
i. . . . . .
~
o
¢
\
I
OT4-Oafter cyclic nitriding. . . . .
-, " ~ OT4-O after cyclic oxycarbonitriding
0,1 -1000
-500
0
500
E [mV]
1000
1500
2000
2500
3000
SCE
Fig. 2, Anodic polarization curves of the corrosion resistance of the Ti(NCO) + Ti2N + ~Ti(N) and TiN + Ti2N + c~Ti(N) type layers produced in isothermal and cyclic plasma nitriding and oxycarbonitriding processes of the OT4-0 titanium alloy and OT4-0 titanium alloy in 1.8 M H:S0~ solution.
(a)
(b)
(c)
(d)
Fig. 3. SEM photographs of the surfaces layers produced on the OT4-0 alloy by isothermal plasma nitriding (a) and oxycarbonitriding (c) and cyclic plasma nitriding (b) and oxycarbonitriding processes (d).
tance to corrosion and to frictional wear of titanium alloys, and thus widens the industrial application range of these alloys. An advantage of this technique is that
by modifying the process parameters, primarily the
temperature and the composition of the gas mixture, we can control the microstructure, the chemical composition and the topography of the surface zone of the growing layer so as to improve its properties. In the
T. llTerzchofi, A. Fleszar / Surface and Coatings Technology 96 (t997) 205-209
208
(a) 20 18
....... ....
16 E
100 MPa 200 MPa
14 12
lO
,,,~ 6
OT4-O after isothermal isc :hermal nitridin(, ni
OT4-O alone
OT4-O after isothermal oxycarbonitridJng -" j."
o
20
4o
6o
loo
ao
time of friction [min]
20 ....
18
100MPa 200MPa
16 14 OT4-O after cyclic oxycarbonitdding
~-, 12 to
¢
.=_
)T4-O alone
~k,
....
,"
"'''"''"
8
..
6
,: n 0
...,:::--" "" " ' 5 g; " " 20
.. . . . . . . . . . .
"'''''''" " 40
T4
i, ....
itr d 60
80
100
time of friction [mini
Fig. 4. Wear resistance of the TiN+Ti2N+~.Ti(N) and Ti(NCO)+Ti:N+~Ti(N) layers produced on the OT4-0 alloy in comparison with OT4-0 titanium alloy.
present study we have found that the properties of the surface layers depend on the carbon, nitrogen and oxygen contents in the layer. At an increased carbon content (up to 8%) the surface hardness of the layer increases to about 2600 HV0.05. An increased oxygen content increases the corrosion resistance, but the layer becomes more brittle, and thus its resistance to frictional wear decreases in comparison with that of the layers subjected to nitriding alone. It should also be noted that the glow discharge activates considerably the process of
layer formation and reduces the treatment time. The cathode sputtering effect occurring during the treatment also enhances the nucleation of titanium nitrides or oxycarbonitrides and promotes the formation of finegrained layers with improved properties. By modifying the microstructure and chemical composition of the surface layers produced on titanium alloys we can improve their performance properties, such as the resistance to frictional wear and corrosion and, it is supposed, also the fatigue strength and the internal stress
T. Wierzchofi, A. Fleszar / Surface and Coatings Technology 96 (1997) 205-209
209
10000
1000
100
E
[3
.,
,
1 c 0.1 a
001 -1000
!
0
1000
2000
E [mV]
3000
4000
5000
SCE
Fig. 5. Anodic polarization curves of TiN + T i , N + ~Ti(N) type layers produced by isothermal and cyclic plasma nitriding before and after the sterilization process in a special solution (containing 0.9%NaC1, anzino acids, vitamins and antibiotics--for example penicillin and streptomycin).
state within the layer. The properties of presented layers developed suggests the possibility of a range of applications acting as biomaterials.
References [I] T. Wierzchofl, J. Sobiecki, Vacuum 44 (I0) (1993) 975. [2] T. Wierzchofl, J. Sobiecki, K. Kurzydlowski, in: O. Hecht, F. Richter, J. Hahn (Eds.), Thin Fihns, DGM Informationgesellschaft, OberurseI, Germany, 1994, p. 195.
[3] M. Zlatanovic, T. Gredic, A. Kltnosic, Surf. Coat. Technol. 63 (1994) 35. [4] E. Rolifiski, Mater. Sci. Engng 100 (1988) 193. [5] Morton, P. H. and Bell, T., Surface engineering of titanium, Memoires et Eludes Scientijiques Rev e de Metalurgie, October, I980, p. 639. [6] J. E. Barry, E. J. Tobin, P. Sioshasi, Surf. Coat. Technol. 51 (1991) i76. [7] T. Wierzchori, J. Michalski, J. Rudnicki, B. Kutakowska, W. Zyrnicki, J. Mater. Sci. 27 (1992) 771. [8] Polish Standard: PN-83/H-04302, The Friction test in 3-toilerscone system, 1983.
CMTINGS ,
ELSEVIER
Surface and Coatings Technology96 (1997) 205-209
HtllNOID6Y
Properties of surface layers on titanium alloy produced by thermo-chemical treatments under glow discharge conditions T. W i e r z c h o f i *, A . F l e s z a r
Warsaw University of Technology, Faculty of Materials Science and Engineering, ul. Narbutta 85, 02-524, Warszawa, Poland Received 20 January 1997; accepted 21 March 1997
Abstract The recent rapid progress in surface treatment techniques dictates that the titanium alloys should have an improved resistance to frictional wear without any loss of their high corrosion resistance. These requirements can be satisfied by producing surface layers of specified microstructure and phase composition. The present paper describes a modification of the plasma discharge nitriding treatment of titanium alloys, i.e. the glow discharge-assisted oxycarbonitriding, which by introducing oxygen, nitrogen and carbon into the surface zone of the layer [a Ti(NCO) type layer] improves its useful properties, primarily the resistance to frictional wear and the resistance to corrosion [1,2]. This is because titanium shows a good affinity to oxygen, carbon and nitrogen, whereas the chemical composition of the layer depends on the chemical composition of the low-temperature plasma that forms under the conditions of glow discharge. The present paper also discusses the results of the corrosion behaviour of the OT4-0 titanium alloy in simulated human body fluids before and after the sterilization process. The isothermal and cyclic plasma nitriding and carboxynitriding processes on titanium alloys can be carried out in a specially modified glow discharge chamber designed for the plasma nitriding process. © i997 Elsevier Science S.A.
Ke)words: Friction; Titanium; Corrosion; Glow discharge chamber
1. Introduction
2. Experimental procedure
That titanium and its alloys are widely used today is associated with the specific physical and chemical properties of this metal. The range of its application is still being widened as the methods of its production are improved so as to control better its properties. Titanium is an attractive structural material, possessing a small specific weight, a very good resistance to corrosion and a high relative strength, but unfortunately, also a low resistance to frictional wear due, among other things, to the formation of adhesive joints and the relatively high friction coefficient. For this reason, various methods of surface treatment are used, such as, for example, glow discharge nitriding, thermal spraying, PVD treatments, anodic oxidation, electroless chemical deposition of metals, ion implantation or laser treatment [3-6]. These techniques are being increasingly used since essentially they improve the properties of products made of titanium and its alloys. Titanium and titanium alloys are widely used today in dentistry and orthopedic surgery because of their good biocompatibility.
Specimens of the OT4-0 titanium alloy (0.4-1.4%A1, 0.5-1.3%Mn, 0.3%Fe, 0.3%Cr, 0.12%Si) were subjected to plasma nitriding and oxycarbonitriding at a temperature of 850 °C for 4 h (isothermal processes) and cyclically between 730 and 930°C for 4 h (between the c ~ + / ~ p transformation temperature). The specimens were subjected to plasma treatment in the gas atmosphere composed of N2 (nitriding) and the vapours of organic compounds +nitrogen (oxycarbonitriding process) at a pressure of about 5 hPa. The layers thus obtained were examined metallographically. The study also included examinations of the phase composition using a Philips 1830 type X-ray difractometer, and wear and corrosion resistance tests. The tribological characteristics of the surface after the treatment were evaluated using the ~three roller-taper' method [7,8]. In this test, friction is applied, under specified conditions, between three fixed cylindrical specimens (rollers) 8 mm in diameter and a rotating conical counter specimen (taper). The linear wear, expressed as the wear depth, was determined by measuring the diameters of the ellipses formed on
* Corresponding author.
0257-8972/97/$17.00 © I997 ElsevierScienceS.A. All rights reserved. PHS0257-8972(97)00113-8
206
Z Wierzchoh, A. Fleszar / Surface and Coatings Technology 96 (1997) 205-209
the surfaces of each roller. The results were then averaged. The counter specimen was made of AISI-45 steel quench hardened and tempered to a hardness of 30 HRC. A constant surface pressure of 100 and 200 MPa was applied. The corrosion resistance test was conducted using the potentiodynamic method in a 1.8 M H2S0 4 solution. The reference electrode was a saturated calomel electrode. Anodic polarization curves were performed at a temperature of 25 °C using a Taccusel PRT-20 potentiostat. Potentiodynamic polarization of the OT4-0 alloy before and after the sterilization process was performed in a special solution (containing inorganic salts, amino acids, vitamins and antibiotics--for example penicillin and streptomycin) using the potentiostat. The sterilization process was conducted in steam at 141 °C and a pressure of 1500 hPa. A temperature of 37 °C, the human body temperature, was maintained during the entire experiment. A saturated calomel electrode (SCE) was used as the reference and a platinium electrode as the counter. The microhardnesses of the polished surfaces of specimen cross-sections were measured using a Vickers indenter under a 50 G load. The specimens were etched using a mixture of HF and HNO3 acids. The etched cross-sections of the specimens were observed using a Nephot-2 light microscope. The surfaces of the layers were observed using a Tesla scanning electron microscope of the BT300 type.
3. Results and discussion
Fig. 1 shows the microstructures of the surface layers produced on the OT4-0 alloy by isothermal and cyclic plasma nitriding and oxycarbonitriding processes. The surface hardnesses of these layers range from 1800 to 1900 HV0.05 after cyclic nitriding and oxycarbonitriding and from 2200 to 2400 HV0.05 after isothermal nitriding and oxycarbonitriding. The nitriding processes produce TiN +Ti2N + c~Ti(N) (a)
(b)
type layers and oxycarbonitriding Ti(NCO)+TizN + c~Ti(N) type layers. The thickness of TiN or Ti (NCO) was about 2 - 4 g m and that of Ti2N was about 35-40 ~m. Fig. 2 compares the measured values of the corrosion resistance of the Ti(NCO)+TizN+c~Ti(N) and TiN+Ti2N+c~Ti(N) type layers produced by isothermal and cyclic plasma nitriding and oxycarbonitriding treatments of the titanium alloy OT4-0 with those obtained for the OT4-0 alloy alone in t.8 M }-I2S04. AS follows from Fig. 2, the best corrosion resistance is shown by the OT4-0 alloy subjected to cyclic temperature glow discharge oxycarbonitriding. This is because the titanium oxycarbonitride layer thus obtained shows the best regularity and homogenity (Fig. 1). The results obtained by scanning electron microscopy (Fig. 3) have shown that the topography of the surface layers varies depending on the process conditions, and thus also depending on the carbon, nitrogen and oxygen contents in the layer. This in turn considerably affects the properties of the surface layers, in particular their resistance to corrosion and frictional wear. Fig. 4 compares the results obtained from examinations of the resistance to frictional wear of the TiN+Ti2N+~Ti(N) and Ti(NCO)+TizN+~Ti(N ) layers produced on the OT4-0 alloy with those obtained for the OT4-0 alone. We can also see that the layers obtained by plasma processes on the OT4-0 titanium alloy have in addition a good resistance to frictional wear. At a load of 100MPa the specimens of the OT4-0 titanium alloy underwent seizure after 10 min. Moreover, the corrosion resistance of the OT4-0 alloy subjected to isothermal and cyclic nitriding has not changed after the sterilization process (see Fig. 5).
4. Conclusions
The plasma nitriding and carboxynitriding techniques developed in the present experiments increases the resis(c)
(d)
Fig. 1. Microstructures of the surfacelayersproduced on the OT4-0 alloy by isothermal plasma nitriding (a) and oxycarbonitriding(c) and cyclic plasma nitriding (b) and oxycarbonitridingprocesses(d).
207
T. Wierzchofi, A. Fleszar / Surface and Coatings Technology 96 (1997) 205-209 10000
OT4-Oafter isothermaloxycarbonitriding . . . . . . . . . . . . . . . . . . . . . . . 10o0 , . ° . . .~. . .
100
,
,"
\
.'/
f
~ after i isothe n g
I,.\.I/-.
oT4-oaone
/ %--
~ "'4'~
//:
?" .--"
..':" .I.."
gO "O
\
\
i. . . . . .
~
o
¢
\
I
OT4-Oafter cyclic nitriding. . . . .
-, " ~ OT4-O after cyclic oxycarbonitriding
0,1 -1000
-500
0
500
E [mV]
1000
1500
2000
2500
3000
SCE
Fig. 2, Anodic polarization curves of the corrosion resistance of the Ti(NCO) + Ti2N + ~Ti(N) and TiN + Ti2N + c~Ti(N) type layers produced in isothermal and cyclic plasma nitriding and oxycarbonitriding processes of the OT4-0 titanium alloy and OT4-0 titanium alloy in 1.8 M H:S0~ solution.
(a)
(b)
(c)
(d)
Fig. 3. SEM photographs of the surfaces layers produced on the OT4-0 alloy by isothermal plasma nitriding (a) and oxycarbonitriding (c) and cyclic plasma nitriding (b) and oxycarbonitriding processes (d).
tance to corrosion and to frictional wear of titanium alloys, and thus widens the industrial application range of these alloys. An advantage of this technique is that
by modifying the process parameters, primarily the
temperature and the composition of the gas mixture, we can control the microstructure, the chemical composition and the topography of the surface zone of the growing layer so as to improve its properties. In the
T. llTerzchofi, A. Fleszar / Surface and Coatings Technology 96 (t997) 205-209
208
(a) 20 18
....... ....
16 E
100 MPa 200 MPa
14 12
lO
,,,~ 6
OT4-O after isothermal isc :hermal nitridin(, ni
OT4-O alone
OT4-O after isothermal oxycarbonitridJng -" j."
o
20
4o
6o
loo
ao
time of friction [min]
20 ....
18
100MPa 200MPa
16 14 OT4-O after cyclic oxycarbonitdding
~-, 12 to
¢
.=_
)T4-O alone
~k,
....
,"
"'''"''"
8
..
6
,: n 0
...,:::--" "" " ' 5 g; " " 20
.. . . . . . . . . . .
"'''''''" " 40
T4
i, ....
itr d 60
80
100
time of friction [mini
Fig. 4. Wear resistance of the TiN+Ti2N+~.Ti(N) and Ti(NCO)+Ti:N+~Ti(N) layers produced on the OT4-0 alloy in comparison with OT4-0 titanium alloy.
present study we have found that the properties of the surface layers depend on the carbon, nitrogen and oxygen contents in the layer. At an increased carbon content (up to 8%) the surface hardness of the layer increases to about 2600 HV0.05. An increased oxygen content increases the corrosion resistance, but the layer becomes more brittle, and thus its resistance to frictional wear decreases in comparison with that of the layers subjected to nitriding alone. It should also be noted that the glow discharge activates considerably the process of
layer formation and reduces the treatment time. The cathode sputtering effect occurring during the treatment also enhances the nucleation of titanium nitrides or oxycarbonitrides and promotes the formation of finegrained layers with improved properties. By modifying the microstructure and chemical composition of the surface layers produced on titanium alloys we can improve their performance properties, such as the resistance to frictional wear and corrosion and, it is supposed, also the fatigue strength and the internal stress
T. Wierzchofi, A. Fleszar / Surface and Coatings Technology 96 (1997) 205-209
209
10000
1000
100
E
[3
.,
,
1 c 0.1 a
001 -1000
!
0
1000
2000
E [mV]
3000
4000
5000
SCE
Fig. 5. Anodic polarization curves of TiN + T i , N + ~Ti(N) type layers produced by isothermal and cyclic plasma nitriding before and after the sterilization process in a special solution (containing 0.9%NaC1, anzino acids, vitamins and antibiotics--for example penicillin and streptomycin).
state within the layer. The properties of presented layers developed suggests the possibility of a range of applications acting as biomaterials.
References [I] T. Wierzchofl, J. Sobiecki, Vacuum 44 (I0) (1993) 975. [2] T. Wierzchofl, J. Sobiecki, K. Kurzydlowski, in: O. Hecht, F. Richter, J. Hahn (Eds.), Thin Fihns, DGM Informationgesellschaft, OberurseI, Germany, 1994, p. 195.
[3] M. Zlatanovic, T. Gredic, A. Kltnosic, Surf. Coat. Technol. 63 (1994) 35. [4] E. Rolifiski, Mater. Sci. Engng 100 (1988) 193. [5] Morton, P. H. and Bell, T., Surface engineering of titanium, Memoires et Eludes Scientijiques Rev e de Metalurgie, October, I980, p. 639. [6] J. E. Barry, E. J. Tobin, P. Sioshasi, Surf. Coat. Technol. 51 (1991) i76. [7] T. Wierzchori, J. Michalski, J. Rudnicki, B. Kutakowska, W. Zyrnicki, J. Mater. Sci. 27 (1992) 771. [8] Polish Standard: PN-83/H-04302, The Friction test in 3-toilerscone system, 1983.