TRIBOLOGICAL BEHAVIORS OF
PLASMA NITRIDED AISI 316 LN TYPE
STAINLESS STEEL IN AIR AND HIGH
VACUUM ATMOSPHERE AT ROOM
TEMPERATURE.
A.DEVARAJU * PhD Research scholar
Department of Mechanical Engineering, Engineering Design division, College of Engineering, Anna University, Chennai-600 025, Tamilnadu, India
Dr.A.ELAYAPERUMAL Assistant professor
Department of Mechanical Engineering, Engineering Design division, College of Engineering, Anna University, Chennai-600 025, Tamilnadu, India
Abstract
In this work, tribological behaviors of the plasma nitrided AISI 316 LN type austenitic stainless steel specimens (both pins and rings) have been analyzed. The experiments have been conducted in high vacuum and in air atmosphere using Vacuum based high temperature Pin-on-disc tribometer. The tribological parameters such as friction coefficient and wear resistance have been analyzed by Origin graphs. The wear mechanisms involved have been identified by recording surface morphology on the wear track and pin surface through scanning electron microscope (SEM) and Optical profilometer. The self mating of AISI 316 LN type stainless steel (316LN) exhibits strong adhesion between the contact surfaces and severe surface damage both in air and in vacuum atmosphere. But, the self mating of Plasma Nitrided 316LN (CrN/CrN) reveals mild wear till the CrN coating peeled off from the pin surface. It has also been proved that Plasma Nitrided (CrN) layer on 316 LN ring was wear resistant layer when it is sliding against the untreated 316 LN pin in air and high vacuum atmosphere.
Key words:pin on Ring configuration; Tribological parameters; adhesion; High vacuum;
1. Introduction
The sliding wear behavior of various steels is a vast subject which has received much attention in recent decades. Tribological behavior of metals which are in sliding contact depends on the many variables such as normal Load, sliding speed, sliding distance, test geometry, surface hardness, surface roughness, working environment, etc., which affect the wear mechanism and wear rate. Many researchers found that the self mating of metals have produced mild wear with the presence of oxide films in air and severe wear with the absence of oxide films in vacuum atmosphere or destruction of these films due to high load or speed[1-5].
*
A.Devaraju, PhD Research scholar, Department of Mechanical Engineering, Engineering Design division, College of Engineering, Anna University, Chennai-600 025, Tamilnadu, India.
The 316 LN materials are very recently used in nuclear industry because of their good corrosion resistance in many environments. In nuclear industry, many critical components inside the reactor which are in sliding contact [6]. When 316 LNs are sliding itself in air atmosphere, it is suffering from severe metallic wear, surface damage and subsurface plastic deformation due to the formation of strong adhesion bonds at the contact junctions [7]. Although austenitic stainless steel works well at high temperatures atmosphere, its components are failed due to sliding wear rather than failure due to high temperature problems or corrosion related problems. Moreover, the metals are in sliding contact in high vacuum atmosphere exhibits tribological problems of high friction, stick -slip motion and high wear, due to stronger adhesion between the surfaces than in air. And it is required high mechanical force or strength to separate the surfaces in contact [8].Adhesive wear was the dominant wear mechanism in vacuum atmosphere since absence of oxygen and moisture in the test environment which usually separates the contact region and act as lubricants. In a high vacuum environment, metals will get atomically clean which exhibit strong adhesive bonds thereby high coefficients of friction and wear when brought into contact [9]. In these situations, it is necessary to overcoat the surfaces of 316LN to eliminate the adhesive wear problem [10].
Chromium nitride coatings are famous to protect from sliding wear problems and corrosion problems. Among many surface coating techniques, it is well known that plasma nitriding offers many advantages over traditional gas nitriding and bath nitriding process to overcoat the surfaces of austenitic stainless steels. Plasma nitriding is a process in which nitrogen is introduced into steel surface at elevated temperatures (greater than 500°C) for improving the surface hardness, wear resistance and corrosion resistance of steels [11-13]. Moreover, plasma nitriding at higher temperature with longer time improves the formation of thick CrN layer and surface hardness. During Plasma nitriding at elevated temperatures, nitrogen penetrates deeper into the surface and subsurface of the sample which is beneficial from the view point of wear resistance since surface hardness is improved along with CrN layer thickness [14-15].The percentage addition of hydrogen in the plasma nitriding process has improved the solubility of the nitrogen in the metal lattice and solid solutions can be obtained. Moreover, the plasma nitrided (CrN) in high hydrogen atmosphere also have increased surface hardness and thickness of the CrN layer [16-18].
Many researchers have done the work on microstructure and mechanical properties of plasma nitrided steel. However, No research work has been done on dry-sliding behavior of Chromium nitride coated 316LN stainless steel under a pin on ring contact configuration in high vacuum atmosphere. Therefore, in this work, the wear resistance of plasma nitrided 316LN at 570°C in a gas mixture of 20%N2–80% H2 for 24hr has been assessed in air and vacuum atmosphere.
2. Materials and methods
The AISI 316LN type stainless steel (316LN) was used as the substrate material in this study. The composition of the material was in wt. % C: 0.02, Ni: 12.1, Cr: 17.9, Si: 0.3, N: 0.07, Mn: 1.8, Mo: 2.4 and Fe: Balance. All the samples were ground using various ranges of Silicon carbide emery paper and then mirror polished using diamond paste to obtain the roughness (Ra) of 0.04µm.
2.1. Plasma nitriding treatment
The substrate specimens were cleaned ultrasonically in acetone for 30min before put into the Plasma nitriding vacuum chamber to deposit the CrN layer.After the samples were placed on the cathode plate, initially the chamber was evacuated to 1x10-2mbar by a rotary pump. And then the below mentioned process parameters have been used to obtain the plasma nitrided (CrN) layer thickness of 60µm – 80 µm for both pins and Rings. The gas flow rate was controlled by a mass flow controller. Plasma nitriding temperature was measured using a Fe-constantan-Fe thermocouple (J type) which was placed at the bottom of the nitriding sample. The Vickers micro hardness measurements reveals a significant increase in the hardness varies from 195HV0.025 (for untreated samples) up to 995 HV 0.025 (plasma nitrided samples).It is five times higher than that of the untreated 316LN metal.
Process Parameters:
Temperature - 570 °C Pressure - 5 mbar Time - 24 Hr Duty cycle - 50 %
2.2. Dry wear test
The working principle of a Pin-on-disc tribometer, as shown in Fig.1, was utilized for evaluating the dry sliding wear resistance of CrN coating at room temperature. The main feature of the test rig is, the stationary pin has been mounted horizontally against a vertically rotating disc which is different from conventional pin on disc tribometer where rotating disc was mounted in horizontal position. The advantage of holding the disc in vertical position is, to eliminate the possibility of trapping the wear debris in the wear track (i.e., third body effect), which alters the wear rate and mechanism. The 316LN was cut from the plate and then machined to the required geometry. The stationary pin sliders were slid against the rotating ring (disc) having the geometry of 92 mm O.D, 72mm I.D and 2mm thickness at 80mm pitch circle diameter with a sliding speed of 0.0753 m/s. The pins rounded with 1.05mm radius at one end were tested. Prior to wear testing; both the disc and the pin samples were cleaned using acetone and then dried for 15min. The disc had been made in the form of ring to reduce the weight of the sample. The Experimental test conditions and results such as Testing environment, Mating Materials, Pin diameter, load, sliding speed, time, temperature and average co efficient of friction have been given the table.1.
Fig.1. Schematic diagram of loading configuration of Pin-on-Disc Tribometer.
The experimental datas were recorded through controller using personal computer which was interfaced to the tribometer using custom based software. Graphs were plotted using the ORIGIN software. Two wear experiments were conducted for each condition to get reasonable reproducibility. The surface morphology of worn surfaces of pins and rings were examined by scanning electron microscope (SEM) and optical profilometer (Non contact type) to identify the wear mechanism and reasons for dynamic co efficient of friction during wear tests.
Table.1.Experimental works results.
Experi -ment name
Testing environment
Mating Materials-(Ring/pin)
Test duration (min)
Pin diameter [mm]
S.D (meter)
Temp (°C)
Normal Load (N)
Sliding speed [m/s]
Average COF
S1 Air CrN /CrN 60 4.0 271 25 11.2 0.0753 0.60304
S2 Air CrN /CrN 10 4.0 45 25 11.2 0.0753 0.64728
S9 Air 316LN/316LN 60 4.0 271 25 11.2 0.0753 0.81454
S10 Air CrN/316LN 60 2.1 271 25 4.1 0.0753 1.10524
C1 High vacuum [1.6x10-4 bar]
CrN/316LN 60 2.1 103 25 4.1 0.0576 1.17007
E1 High vacuum [1.6x10-4 bar]
316LN/316LN 60 2.1 103 25 4.1 0.0576 2.59206
[Note; 1.CrN coating thickness was 60µm – 80 µm for both pins and Rings.
2. Two experiments have been conducted for each condition to obtain reasonable reproducibility.] Direction of
load
WearTrack
Falling of weardebris
Debriscollection tray
Ring
3. Results and discussion
3.1. Plasma nitrided 316LN (CrN) Ring against Plasma nitrided 316LN (CrN) Pin in air-CrN/CrN
The wear resistance of plasma nitrided (CrN) 316LN has been assessed by mating itself (CrN coated pin against CrN coated Ring) in air and its result has been compared with the result of self mating of untreated 316LN.The CrN coated Pin (4mm diameter) against CrN coated ring wear test was conducted for the selected constant sliding speed of 0.0753 m/s, normal load of 11.2N and at temperature of 25C in air
.
It’s results shows that the co efficient of friction increases slowly from 0.25 to 0.75 without much fluctuations between the sliding distance of 0m and 101m.But,the sliding distance between 101m and 271m, the co efficient of friction fluctuate deeply and maintains the average COF of 0.81(Fig.2(a)).The pin displacement towards the ring was 40 µm between the sliding distance 0m and 50m.After that the pin displacement was increased slowly from 40 µm to 70 µm as the sliding distance increases from 50m to 101m.But, pin displacement is increased rapidly upto 130 µm between the sliding distance of 101m and 110m.Then, it is maintaining steady at 140 µm as the sliding distance increases from 110m to 271m (Fig.2 (b)).The reason for the lower friction co efficient upto the sliding distance of 101m is due to the sliding contact surfaces were between CrN against CrN either purely or partly. And the fluctuations of friction co efficient are also low for the same reason since CrN/CrN contacts avoid the strong adhesion between the contact surfaces. But, beyond 101m sliding distance, the co efficient of friction fluctuate deeply because of CrN coating peeled off from the pin contact surface (Fig.3(b)) and thereby substrate material(ie., 316LN) was directly contacted over the CrN layer of the ring surface. Therefore, the substrate materials of the pin have been transferred to the CrN ring surface since it is softer than the ring CrN surface (Ra =3.83). Fig.3 (a) shows the transferred pin material on the wear track of the ring. When sliding continues, the sliding contact occurs between 316LN material of pin surface and the transferred material of 316LN on the wear track. Then, it became untreated metals mating ie., 316LN/316LN.Therefore, the strong adhesion between the pin and ring at the contact region due to mating of similar metals in sliding contact. This could be the reason; the friction co efficient also increases upto 0.9 along with deep fluctuations. As long as CrN coating layer is with the pin surface, it avoids the strong adhesion between contact regions and maintains the lower friction coefficient. Once CrN coating layer is peeled off from the pin contact surface, the pin displacement increases rapidly upto130 µm at 101m sliding distance. After that the pin displacement maintaining 140 µm since sliding occurs between 316LN material of pin surface and the transferred material of 316LN on the wear track.
0 50 100 150 200 250 300
0.0 0.5 1.0 1.5
C
o
ef
fi
cient of
f
ri
ction
Sliding Distance(m) Fig.2(a).S1-CrN/CrN
0 50 100 150 200 250 300
20 40 60 80 100 120 140 160
Sliding Distance(m) Fig.2(b).S1-CrN/CrN
Pin M
ove
m
ent
tow
a
rd
s R
in
g
(m
ic
ron
)
Figure.3. The scanning electron micrograph (SEM) of CrN coated pin sliding against CrN coated disc at 25°C in air (a)wear track (b) Pin surface.
0 10 20 30 40 50
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
C
o
e
ffi
ci
en
t o
f
fr
ic
ti
o
n
Sliding Distance(m) Fig.4.S2-CrN/CrN
Figure.4. Friction curve of CrN coated pin sliding against CrN coated disc at 25°C in air for shorter sliding distance.
Figure.5. The scanning electron micrograph (SEM) of CrN coated pin sliding against CrN coated disc at 25°C in air for shorter sliding distance (a) Pin surface at lower magnification (b) Pin surface at higher magnification.
It has been conducted a separate experiment for the shorter sliding distance of 47m(10min) to identify the failure of CrN coating on the pin contact surface during sliding contact against the CrN coated ring in air. The CrN Coating failure starts at 25m sliding distance itself under the selected conditions which conforms the larger dynamic
3(a) 3(b)
AISI 316LN
Transferred pin materials
Surface cracks AISI 316LN
CrN layer
friction curve at 25m(Fig.4 ).Even for the shorter sliding distance(47m), the coatings have been peeled from the pin surface for small area (Fig.5 (a)).Although the CrN coatings starts to peeled off from the surface at 25m sliding distance, it completely peeled off only at greater the sliding distance of 101m.Figure.5(b) shows the brittle fracture mode of failure was occurred on the pin surface which evidenced that more surface cracks are available. However, CrN coatings have not peeled at any point of the wear track rather it contains only transferred material from pin surface (Fig.3 (a)). Therefore, the CrN coatings layer on the ring surface have been proved that wear resistant layer whereas coatings was peeled off only from the contact surface of pin since it contacts at a point during the sliding wear tests. However, as long as CrN against CrN contacts, it avoids the strong adhesion between contact regions and maintains the lower friction coefficient. However, further research is required to find out wear resistance of CrN layer for pure CrN/CrN sliding test. Moreover, it is concluded that Pin on ring (disc) configuration is not suitable to check the wear resistance of CrN layer for pure CrN/CrN couples for longer sliding distance.
An experiment was conducted for the untreated 316LN/316LN couples to compare the tribological behaviors of CrN/CrN couples results. The 316LN/316LN couples exhibits high friction along with deep fluctuations from the beginning stage of the test itself and the average COF of 0.81 as shown in Figure 6(a).The 316LN pin displacement towards 316LN ring sharply increases upto 500 µm. After that, it increases proportional to the sliding distance as shown in Fig. 6(b).The 2D profilometry image on the wear track of the 316LN/316LN couples shows wider and deeper(Fig.7) which reveal that more material loss from pin and ring. As mentioned earlier, the reason for the deep fluctuations in the friction curve could be due to strong adhesion between the contact regions of 316LN/316LN couples since similar material (especially iron based metals) mated in sliding contact will exhibit strong adhesion. The material loses of both pin and ring was high for the same reasons. But, we could found that no materials have been lost from the CrN ring during the CrN/CrN test whereas wear track contains only transferred material from the pin. Therefore, CrN coating layers on the substrate metal proves that wear resistant layer.
0 50 100 150 200 250 300
0.25 0.50 0.75 1.00 1.25 1.50
Fig.6(a).S9-316LN/316LN
C
o
ef
fi
ci
ent of
f
ri
ct
ion
Sliding Distance(m)
0 50 100 150 200 250 300
0 200 400 600 800 1000 1200
Pi
n
M
ovem
ent
t
o
w
a
rds
R
ing
(m
icro
n)
Sliding Distance(m) Fig.6(b).S9- 316LN/316LN
Figure.6. The untreated 316 LN/316LN couples at 25°C in air. (a) Friction curve and (b) Pin displacement towards ring
3.2. Plasma nitrided 316LN (CrN) ring against untreated 316LN pin in air and in high vacuum
The wear resistance of plasma nitrided (CrN) coatings have been assessed in air and in high vacuum by mating CrN coated ring against untreated 316LN pin ( ie 316LN pin against CrN coated Ring) and its result has been compared with the result of untreated 316LN/316LN couples in high vacuum.
The 316LN Pin (2.1mm diameter) against CrN coated ring wear test was conducted for the selected constant sliding speed of 0.0753 m/s, Normal load of 4.1N and at temperature of 25C in air atmosphere which result shows the lower friction co efficient of 0.5 upto the sliding distance of 30m.After that friction co efficient is increased slowly upto 1.5 along with deep fluctuations as shown in Fig.8 (a). Similarly, the pin displacement towards ring maintains steadily upto 30m sliding distance. After that, it also increases proportional to the sliding distance (Fig.8 (b)). The reason for lower COF and steady pin displacement upto the sliding distance of 30m was, sliding occurs between CrN coated ring against 316LN pin (ie., Pure CrN/316LN) and upto 30m sliding distance there is no transfer material from pin to the wear track. But, beyond 30m sliding distance, the pin material starts to transfer to the wear track of ring. Therefore, sliding occurs between transferred pin materials (either partly or fully) to the wear track and pin. Then, it became 316LN/316LN.This could be the reason for increase of friction coefficient from 0.5 to 1.5 along with deep fluctuations because of strong adhesion at the contact region since similar material exhibit strong adhesion. It results more metal loss from pin. However, there is no metal loss from CrN ring whereas it contains only transferred material.
An experiment conducted on CrN coated ring against 316LN pin in high vacuum atmosphere which result shows the higher initial friction co efficient of 1.75.However, the sliding distance beyond 30m, the friction co efficient reduced to 1.2 (Fig.9(a)) which is similar approach in air atmosphere for the same test conditions. Although the friction is very high during initial state, the pin displacement maintains steady position like in air (Fig.9 (b)).
0 50 100 150 200 250 300
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Co
ef
fi
ci
en
t
of
f
ri
cti
o
n
Sliding Distance(m) Fig.8(a).S10-CrN/316LN
0 50 100 150 200 250 300
0 100 200 300 400 500
P
in
M
o
vem
ent
t
o
ward
s Rin
g
(m
ic
ron
)
Sliding Distance(m) Fig.8(b).S10-CrN/316LN
0 20 40 60 80 100 0.0
0.5 1.0 1.5 2.0 2.5 3.0
C
o
ef
fi
ci
en
t
of
f
ri
cti
on
Sliding Distance(m) Fig.9(a).C1-CrN/316LN
0 20 40 60 80 100
0 20 40 60 80 100
Sliding Distance(m) Fig.9(b).C1-CrN/316LN
P
in M
o
vem
ent towards Ring
(m
ic
ron)
Figure.9. untreated 316LN pin sliding against CrN coated ring at 25°C in high vacuum atmosphere (a) Friction curve and (b) Pin displacement towards ring.
Fig.10. The scanning electron micrograph (SEM) on the wear tack of CrN coated ring tested against 316LN pin sliding at 25°C (a) in air and (b) in high vacuum.
The reason for very high friction co efficient in the initial state of the experiment should be due to strong adhesion between pin and ring because of absence of contaminants such as moisture, humidity, dirt etc in high vacuum atmosphere. However, when sliding continues, the pin material is transferred to the ring and friction reduces to 1.2 which is similar approach of air atmosphere. The scanning electron micrograph (SEM) on the wear tack of CrN coated ring tested against 316LN pin sliding at 25°C in air atmosphere was smooth(Fig.10(a)) comparatively whereas in vacuum atmosphere it appeared some adhesive marks along with smooth surface(Fig.10(b)). The reason for adhesive marks in vacuum atmosphere tests was the stronger adhesion between the contact surfaces than in air atmosphere.
It has also been conducted an experiment for the 316LN/316LN couples in high vacuum atmosphere to compare its result with CrN/316LN couples in high vacuum atmosphere results. The 316LN/316LN couples produces high friction along with deep fluctuations and an average COF of 2.6 and initial friction is too high(3.25) as shown in Fig. 11(a).The 316LN pin displacement towards 316LN ring sharply increases upto the sliding distance of 40m. After that, it maintains the steady state as shown in Fig.11 (b).As mentioned earlier, the reason for high friction and deep fluctuations in the friction curve could be due to strong adhesion since similar material pair exhibit strong adhesion. Apart from this, in vacuum atmosphere, the adhesion will be stronger than in air atmosphere since contact surfaces are atomically clean which results slip stick mechanism will comes into picture. Therefore, the material loses of both pin and ring also very high for the same reasons. But, we could found that no materials or negligible amount of materials have been lost from the CrN ring during the CrN/316LN test both in air and high
10(a) 10(b)
Smooth Surface
vacuum atmosphere whereas wear occurs only on the 316LN pin. Therefore, CrN coating layers on the ring surface proves that wear resistant layer both in air and vacuum atmosphere.
0 20 40 60 80 100 120
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
C
o
ef
fi
ci
en
t of
f
ri
cti
on
Sliding Distance(m) Fig.11(a).E1-316LN/316LN
0 20 40 60 80 100 120
0 50 100 150 200 250 300 350 400
Sliding Distance(m)
P
in M
ovem
en
t
toward
s R
ing
(m
icron) Fig.11(b).E1-316LN/316LN
Fig.11. The self mating of untreated austenite stainless steel AISI 316 LN type (like on like) at 25°C in vacuum atmosphere. (a) Friction curve and (b) Pin displacement towards ring
4. Conclusion
The wear resistance of plasma nitrided (CrN) coatings have been assessed by mating itself (CrN coated pin against CrN coated Ring) in air. Its result exhibits
Lower friction coefficient as long as CrN coating layer is with the pin surface since CrN/CrN contacts avoid the strong adhesion between the contact surfaces.
CrN coatings of pin contact surface have failed at 101m sliding distance and therefore pin substrate material(316LN) have been transferred to the wear track of ring. Then, the sliding contact became 316LN/316LN.
However, the CrN coatings layer on the ring surface have been proved that wear resistant layer whereas wear occurred only on pin surface since it contacts at a point during sliding wear tests and therefore coatings was peeled from the contact surface of pin. Pin on ring (disc) configuration is not suitable to check the wear resistance of CrN layer for pure CrN/CrN sliding test for longer sliding distance.
whereas result of untreated 316LN/316LN in air atmosphere shows deep fluctuations in the friction curve and high wear since similar material (especially iron based metals) mated in sliding contact will exhibit strong adhesion.
The wear resistance of plasma nitrided (CrN) coatings has also been assessed by mating 316 LN pin (316 LN pin/CrN ring) in air and vacuum atmosphere.
CrN coated ring against 316LN pin in air atmosphere produces lower friction co efficient of 0.5 upto the sliding distance of 30m because of pure CrN/316LN.when sliding increases further, pin coating failed and then 316LN has transferred to the wear track of ring and the sliding contact became 316LN/316LN.
CrN coated ring against 316LN pin in high vacuum atmosphere produces high initial friction because the absence of moisture, humidity, dirt etc between the contact surfaces. However, when sliding increases further, it produces similar approach in air atmosphere.
5. Acknowledgements
We express our sincere thanks to Dr.S.venugopal, Indira Gandhi center for atomic research (IGCAR), kalpakkam, for providing specimen materials of Rings and pins. We wish to thank our Prof. Sathis vasu kailas, Indian Institute Science, Bangalore, for permitting us to do the experimental works in vacuum based pin on disc Tribometer and for his valuable suggestions & support in carrying out this work.
References
[1] Lim, S. C.; Ashby, M. F. (1987): Overview No. 55, Wear mechanism maps, Acta Metall, 35,pp. l-24.
[2] Lim, S. C.; Ashby, M. F.; Brunton, J. H. (1987): Wear-rate transitions and their relationship to wear mechanisms, Acta Metall, 35, pp. 1343-1348.
[3] Smith, A. F. (1984): The friction and sliding wear of unlubricated 316stainless steel at room temperature in air, Wear, 96, pp. 301 - 318
[4] Smith, A. F. (1986):The friction and sliding wear of unlubricated 316stainless steel in air at room temperature in the load range 0.5 - 90 N, Wear, 110, pp. 151 - 168.
[5] Smith, A. F. (1986): Influence of environment on the unlubricated wear of 31 6 stainless steel at room temperature, Tribology international, 19, pp. 1-10.
[6] Bhaduri, A.K.; Indira ,R.; Albert ,S.K.; Rao, B.P.S.; Jain, S.C.; Asokumar ,S. (2004) :Selection of hardfacing material for components of the Indian Prototype Fast Breeder Reactor, J. Nucl. Mater., 334 ,pp. 109-114.
[7] Hsu, K.L.; Ahn, T.M.; Rigney, D.A.. (1980): Friction, wear and microstructure of unlubricated austenitic stainless steels, Wear, 60, pp. 13-37.
[8] Kazuhisa Miyoshi. (1999): Foreword-Considerations in vacuum tribology (adhesion, friction, wear, and solid lubrication in vacuum), Tribology International, 32, pp. 605-616.
[9] Buckley, DH. (1981): Surface effects in adhesion, friction, wear and lubrication, Elsevier Book Series, Elsevier Scientific Publishing Co., vol. 5. pp.206-210.
[10] Sun, Y.; Bell, T. (2002) : Dry sliding wear resistance of low temperature plasma carburised austenitic stainless steel, Wear, 253, pp. 689–693.
[11] Larisch, B.; Brusky, U.; Spies, H.J. (1999): Plasma nitriding of stainless steels at low temperatures, Surf. Coat. Technol, 116, pp. 205-211.
[12] Musil, J.; Vlcek, J.; Ruzicka, M. (2000): Recent progress in plasma nitriding, Vacuum, 59, pp. 940-951.
[13] Borges, C.F.M.; Hennecke, S.; fender, E. (2000): Decreasing chromium precipitation in AISI 304 stainless steel during the plasma-nitriding process,Surf. Coat. Technol, 123, pp. 112-121.
[14] Menthe, E.; Rie, K.T.; Schultze, J.W.; Simson, S. (1995): Structure and properties of plasma-nitrided stainless steel, Surf. Coat.Technol, 74, pp. 412-416.
[15] Baggio-Scheid, V.H.; Abdalla, A.J.; Vasconcelos, G. de. (2006): Effect of heating post-treatment on nitrided stainless steel, Surf. Coat.Technol, 201, pp. 4058-4061.
[16] GRILL, A.; ITZHAK, D.( 1983):Nitriding of AISI M2 tool steel in an inductive R.F. plasma,Thin Solid Films, 101, pp. 219-222. [17] Kumar, S..; Baldwin, M.J.; Fewell, M.P..; Haydon, S.C.; Short, K.T.; Collins, G.A..; Tendys, J. (2000): The effect of hydrogen on the
growth of the nitrided layer in r.f.-plasma-nitrided austenitic stainless steel AISI 316 Surf. Coat.Technol, 123, pp. 29-35.
