G. Bregliozzi

The influence of atmospheric humidity and grain size on the friction and wear of AISI 304 austenitic stainless steel

The tribological properties of a ultra-fine AISI 304 austenitic stainless steels obtained by means of a martensitic transformation and subsequent austenite reversion are reported. The effects of the grain size on the wear resistance of such material is, for the first time, investigated as a function of the atmospheric humidity. Decrease of relative humidity in wear tests of AISI 304 steel produces an increase in weight loss and in the friction coefficient. A beneficial effect of grain refining is also shown with respect to large grain steel in that the finer grain steel produces less initial weight loss and the weight loss with an increase in the humidity is also less pronounced.

Microfrictional properties of diamond-like carbon films sliding against silicon, sapphire and steel

Abstract The low contact pressure characteristic of the microtribological regime relative to macro and nanosystems is suited for testing the microfrictional properties of different types of thin films. Motivated by macro as well as microsystem applications, this study investigates the microfrictional properties of different types of diamond-like carbon (DLC) films, prepared using low- and high-frequency plasma-assisted chemical vapor deposition (HF-PACVD) and the vacuum arc method. Testing was performed with a reciprocating precision microtribometer. Silicon, sapphire and steel balls were used as counterbodies. Friction–load curves suggest that, for applied forces in the μN to mN regime, two properties have a strong influence on the microfriction: first, the chemical composition plays a dominant role and second, the film roughness. With silicon and steel balls, the microfriction of hydrogen-free DLC films was greater than the hydrogen-containing films. With sapphire counterbodies, the results indicate that microfriction is inversely proportional to the film roughness. Also, for the films tested, microfriction was determined to be independent of the sliding velocity. For the force (pressure) regimes tested, mild wear was observed on silicon and some steel counterbodies, while no wear could be detected on any of the DLC films. These results illustrate the utility of implementing microtribological testing in comparative coating studies.

Fluorinated amorphous carbon thin films: Analysis of the role of the plasma excitation mode on the structural and mechanical properties

The effects of pulsed plasma enhanced chemical vapor deposition on the thermally induced gas effusion characteristics of fluorinated amorphous carbon thin films are investigated. The main contributions to the effusion spectra are found to come from hydrogen, hydrocarbons, and CF4. With a plasma excitation frequency of 10−3 s, the film produced is relatively compact and the effusion of hydrogen-related species dominate. A strong change in the effusion characteristics for the highest on-time plasma excitation (10−1 s) indicates that an interconnected network of voids is present. Strong effusion of CF4 related species is in fact found to be consistent with a surface desorption process and can only be observed when the void network dimensions are large enough. Nanoindentation measurements showed increased elasticity of the film, as well as increased hardness upon reduction of the plasma excitation period. Raman spectroscopy was applied to corroborate the effusion results, indicating a structural transition fr...

Influence of atmospheric humidity and grain size on the friction and wear of high nitrogen austenitic stainless steel

Research on steels is still active and is motivated by the need to make further improvements in their properties. Among the different steel types, austenitic stainless steels possess good corrosion resistance and formability. However, they also have a relatively low yield strength. One method of increasing the yield strength is by alloying the steel with nitrogen. Such nitrogen-alloyed austenitic stainless steels exhibit attractive properties such as high strength and ductility, good corrosion resistance and reduced tendency to grain boundary sensitation [1]. The high austenitic potential of nitrogen allows the reduction of nickel content in steel. This offers additional advantages such as cost savings and makes the steel more suitable for stainless steel applications involving contact with human skin for people with nickel allergies. Due to the above-mentioned properties, in recent years significant efforts have been devoted to the production of high nitrogen austenitic stainless steels, and metallurgists have been very active in research concerning this class of steels. Particularly, considerable attention has been paid to solidification mechanisms and recrystallization processes [2–4]. Nitrogen in solid solution produces, in the austenitic stainless steel, many desirable properties: it stabilizes the austenite γ phase and increases the resistance to intercrystalline and pitting corrosion, but the most important property is the increase of the yield strength (RP0.2) without a corresponding decrease in ductility. There is another way of increasing the yield strength without severely affecting the ductility. This can be achieved by grain refining. Since austenitic stainless steels do not undergo phase transformation at typical annealing temperatures, the only way to refine the grain size is by recrystallization after cold rolling. An alternative way to obtain fine grains is by applying an austenite-martensite-austenite transformation. In previous publications [5, 6], we examined the influence of grain size and chemical composition on the mechanical properties and corrosion resistance of this family of steels.

Cavitation Erosion and Friction Behavior of Stainless Steel as a Function of Grain Size

Research conducted on steels is motivated by a technological need to further improve their properties. Among the different steel types, austenitic stainless steels possess good corrosion resistance and formability. However, they also have a low yield strength. One way of increasing the yield strength is by grain refining. This work presents a study on the cavitation erosion and friction behavior of AISI 304 austenitic stainless steel characterized by two different grain sizes: 2.5 μm and 40 μm. The cavitation erosion behavior in water with different pH values and at room temperature has been studied by using a 20 kHz ultrasonic vibratory apparatus. The grain size of the steels has an important effect on the nature of damage produced on the surface of the samples. The resistance to cavitation erosion increases with decreasing grain size. It was also found that cavitation erosion resistance of the two steels is sensitive to variations in the pH value; decrease of this value produces an increase in surface damage. Using a precision microtribometer, with applied loads in the μN regime, it was found that capillarity plays a dominant role. At the same loads, in high humidity environments, both the fine and coarse grain steels exhibit high friction relative to measurements performed under dry conditions. At high loads (20 mN and above) a reversal in microfrictional behavior occurs in that friction is higher under dry conditions than under moist conditions.

A comparative microtribological investigation of diamond-like carbon films for applications in microsystems

This study reports on the microfrictional properties under reciprocating sliding of different types of commercially available diamond-like carbon (DLC) films, prepared using low and high frequency plasma-assisted chemical vapor deposition (PACVD) and the vacuum arc method. Silicon and sapphire balls were used as counterbodies. Friction-load curves suggest that, for applied forces in the N to mN regime, two properties have a strong influence on the microfriction: First, the chemical composition plays a dominant role and second, the film roughness. With silicon against DLC, microfriction of hydrogen-free DLC films was greater than the hydrogen containing films. With sapphire counterbodies, microfriction was inversely proportional to film roughness. For all films tested, microfriction was independent of the sliding velocity. Mild wear in the form of scratches was observed only on silicon, while no wear could be detected on any of the DLC films. These results illustrate the utility of implementing microtribological testing in a coating development for microsystems.