Juha-Pekka Hirvonen

Surface structure and properties of ion-nitrided austenitic stainless steels

The near surface structure and nitrogen concentration of the low-temperature low-pressure ion-nitrided stainless steels (SS) was studied by using X-ray diffraction (XRD), transmission electron microscopy (TEM), nuclear resonance broadening (NRB) and microhardness techniques. The surface nitrogen content as determined by NRB was found to increase with nitriding time such that at long nitriding times the surface nitrogen concentration was higher than for any equilibrium nitride in the FeN system. Nitrogen contents were slightly higher for type-304 than for type-316 stainless steels. Simultaneously with increasing surface nitrogen content, a strong shift and broadening of X-ray diffraction peaks occured. In the surface of the nitrided layer expanded austenite as well as e-phase analogous to e-martensite is formed. At long nitriding times (high nitrogen surface contents) the structure of the surface corresponds to cubic MN1-x nitride. At intermediate nitriding times (and nitrogen contents) possibly some e′-nitride is also formed. TEM study of the surface layer showed that after long nitriding times the expanded austenite with occasional weak primitive lattice reflections was the dominating phase and the hexagonal e-phase was habited as thin platelets on the (111) planes of the nitrogen supersaturated austenite. The hardness of the compound layer can be as high as 25 GPa. The high hardness is suggested to result from nitrogen supersaturation, high dislocation density and thin platelets of e-phase in the surface of the compound layer.

Tribological characteristics of diamond-like films deposited with an arc-discharge method

Using an are-discharge method, we deposited a diamond-like carbon film 600 nm thick on hardened steel. Characterization of the film was carried out with Raman spectroscopy. In dry sliding wear and friction tests, with a hardened steel pin as a counterpart, we obtained a friction coefficient between 10000 and 20000 cycles, with the maximum value of 0.18. The value decreased to 0.12 after about 100000 cycles. We obtained a wear coefficient of 7 × 10−17 m3/mN. A transfer layer formed on the pin during sliding and probably had the dominating effect on the tribological behavior. We observed in nanoindentation measurements that the film softened in a wear track during the first 20000 cycles. Although fracture pits on the wear track occurred, fracture is not the dominant failure mechanism of these films. Degradation of good tribological properties was caused mainly by partial wear-through of the film after 370000 cycles and by a subsequent redeposition of the transfer film on the wear track during prolonged sliding.

Comparison of diamondlike coatings deposited with C+ and various hydrocarbon ion beams

The mechanical properties of the diamondlike coatings deposited with mass‐separated C+, CH+3, CH+4, and C2H+2 ion beams have been compared. The hardness, abrasive wear resistance, and adhesion of the coatings prepared with the C+ ion beam were superior to those of the coatings prepared with other ions. The most serious drawback of the films prepared with hydrocarbon beams was their brittleness and weak adhesion.The mechanical properties of the diamondlike coatings deposited with mass‐separated C+, CH+3, CH+4, and C2H+2 ion beams have been compared. The hardness, abrasive wear resistance, and adhesion of the coatings prepared with the C+ ion beam were superior to those of the coatings prepared with other ions. The most serious drawback of the films prepared with hydrocarbon beams was their brittleness and weak adhesion.

Characterization of the mechanical properties of carbon metal multilayered films

Abstract Hard amorphous carbon films exhibit excellent wear resistance and low friction. The hard carbon films are brittle and have a high internal stress. Also the abrupt change of elastic modulus at the interface of the film and metallic substrate results in low adhesion and reduces load carrying capacity. Composite film structures with alternating layers of hard carbon and metallic films has been shown to possess unique mechanical properties such as enhanced fracture toughness and high hardness. Elastic mismatch may also be avoided by careful control of composition. The stress relaxation, when possible, enables the growth of thicker films. Multilayer films with alternating layers of hard carbon and TiN x have been deposited by using an arc discharge deposition apparatus. The carbon plasma is generated with a pulsed plasma source. The titanium and TiN films are deposited by using a d.c. arc source equipped with a particle filtering. Films with a thickness of about 0.5 μm were deposited with 10 layers of carbon and TiN x . The substrate materials were AISI440B stainless steel and (100) silicon. The film composition was determined by nuclear resonance analysis, Rutherford backscattering, scanning electron microscopy and scanning force microscopy. The wear resistance and load carrying capacity was tested by using a pin-on-disc test. The friction coefficient of the multilayer films was observed to be lower than for the pure diamond-like carbon films, while the wear rate of the multilayer film slightly increased and the wear rate of the counter surface was increased about ten fold.

Tribological characterisation of hard carbon films produced by the pulsed vacuum arc discharge method

Abstract Hard diamond-like carbon (DLC) films were deposited on silicon and high-speed steel substrates using a pulsed vacuum arc discharge method. The plasma plume was focused on the substrate using a direct electromagnetic coil. Several methods were used for coating characterisation. The film composition was analysed using Rutherford backscattering spectroscopy and forward recoil spectroscopy. About 0.5 at.% oxygen and about 1 at.% hydrogen was detected in the film. The tribological properties of the carbon films were studied using pin-on-disc tests. The counterface materials employed were alumina and hardened steel (AISI 52100 and M50) pins, which were slid against the coated substrates. The friction coefficient was measured and the wear surfaces were studied. The sliding speed was in the range 0.02–0.6 m/s and the load in the range 5–20 N. The tests were carried out in air with a relative humidity of 50±2% and at a temperature of 24±3 °C. The test results show that the DLC coatings produced for this study generally had a coefficient of friction (μ) of about 0.2. The lowest value measured was μ =0.14. The wear resistance of the coatings was good, provided that the adhesion to the substrate was sufficient. The comparative tests with titanium nitride and titanium aluminium nitride coatings showed that DLC films are considerably more wear resistant than titanium-based coatings.

Density measurements of diamond-like coatings using a low energy accelerator

Abstract With the aid of nuclear physical methods based on the use of a low energy accelerator it is shown that the density of even a small, uneven and rough diamond-like coating prepared by mass-separated ion beam deposition can be determined reliably. The density of the coating corresponds to that of diamond. Outlines of the used methods, i.e. Rutherford backscattering and/or nuclear resonance shift combined with the use of a profilometer and Doppler shift attenuation, are presented.

Interfacial characteristics of arc-discharge-deposited diamond-like films on 19 different substrate materials

Abstract The pulsed-arc-discharge method has been used to deposit diamond-like carbon (DLC) films on 19 different substrate materials such as metals, ceramics and polymers. The film thickness was about 0.2 μm. The adhesion of the DLC to the substrate was evaluated using pull tests, the Vickers indentation method and scratch tests. The pull test results have been used to estimate the adhesion quantitatively using the critical load obtained from the scratch tests. In general, materials with a high affinity for carbon also have the best adhesion. The interfaces have been examined using ion beams by measuring the detached surface after a pull test on the DLC film. Larger amounts of substrate material can be detected on the pulled DLC film with materials which form stable carbides.

Corrosion properties of amorphous Mo-Si-N and nanolayered Mo-Si-Nn/SiC coatings

Corrosion properties of sputter deposited MoSi{sub 2}, SiC, Mo-Si-N (MoSi{sub 2.2}N{sub 2.5}) and nanolayered Mo-Si-N/SiC coatings on Fe37 low carbon steel have been studied using electrochemical polarization measurements in 1 N H{sub 2}SO{sub 4} solution. A decrease in both critical current density for passivation and minimum current in passive state was observed in annealed nanolayered Mo-Si-N/SiC coating compared to each of its constituents alone as single layer coating. On contrary to MoSi{sub 2} coating, only slight increase in critical current density was observed in Mo-Si-N coated sample after annealing. Molybdenum disilicide source material has good thermal and electrical conductivity, which allows effective dc-magnetron sputter deposition. Therefore this is a relatively simple method to produce amorphous coatings which have a high crystallization temperature and promising properties for corrosion applications.

Nanolayered gradient structures as an intermediate layer for diamond coatings

Abstract A molybdenum/titanium multilayered film was used as an intermediate film between a diamond-like carbon (DLC) coating and steel and silicon substrates. The elastic properties of the Mo/Ti intermediate layer were modified by using a gradient thickness of the titanium films. An attempt was thus made to obtain a smooth gradient of the elastic modulus from the substrate (steel, 210 GPa) to the DLC film (300 GPa). The Mo/Ti multilayer was deposited in a sputter deposition unit with two sputter targets and a rotating substrate holder. The hydrogen-free DLC coating was deposited with a pulsed arc discharge method. The Mo/Ti film was characterized by several sophisticated methods, such as cross-section scanning force microscopy, cross-section transmission electron microscopy. Rutherford Backscattering spectroscopy and nano-indentation. The film has a layered structure with a columnar growth. Preliminary tests of the mechanical properties were performed by using a scratch test. On silicon the Mo/Ti film detached at a slightly higher critical load compared with the DLC film with no intermediate film. However, poor adhesion of the Mo/Ti film to the steel substrate prevented evaluation of the mechanical properties of the sample.

Friction and wear of polytetrafluoroethylene on diamond-like carbon film

Both diamond-like carbon (DLC) film and polytetrafluoroethylene (PTFE) are materials endowed with excellent tribological behaviour. These two materials have the similar property of self-lubricity which produces an extremely low coefficient of friction (COF). We present the first tribological results for PTFE sliding on a DLC film