In the present study, AISI 8620, 52100 and 440C steels were plasma paste boronized (PPB) by using 100% borax paste. PPB process was carried out in a dc plasma system at temperature of 700 and 800°C for 3 and 5 h in a gas mixture of 70%H 2 -30%Ar under a constant pressure of 4 mbar. The properties of boride layer were evaluated by optical microscopy, X-ray diffraction and Vickers micro-hardness tester. X-ray diffraction analysis of boride layers on the surface of the steels revealed FeB and Fe 2 B phases for 52100 and 8620 steels and FeB, Fe 2 B, CrB and Cr 2 B borides for 440C steel. PPB process showed that since the plasma activated the chemical reaction more, a thicker boride layer was formed than conventional boronizing methods at similar temperatures. It was possible to establish boride layer with the same thickness at lower temperatures in plasma environment by using borax paste.

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Thermo-reactive diffusion chromizing followed by pulsed plasma nitriding were carried out on AISI 52100 and 8620 bearing steels. The chromized samples were pulse-plasma nitrided for 5 h at 500°C in various N 2-H 2 gas mixtures. The coated steels were characterized using scanning electron microscopy, X-ray diffraction and microhardness testing. The unlubricated wear behaviors of only chromized and duplex treated steels were investigated in ball-on-disc system tests at room temperature. X-ray diffraction patterns of the duplex treated samples containing H 2 indicated the formation of dominant CrN and Cr 2 N nitrides as well as the formation of Cr 3 C 2 and Cr 7 C 3 carbides. Gas mixtures in the plasma nitriding, which was performed after chromizing, have a significant influence on the wear rate of the duplex treated steels. The wear and friction tests showed that the lowest friction coefficient and wear rates were observed for the samples duplex treated in a 50%N 2 + 50%H 2 plasma. Conversely, the lowest wear resistance was observed on the samples duplex treated in a gas mixture of 75%N 2 + 25%H 2 , probably due to formation of a hard and brittle layer.

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In this work, AISI 316 steel was plasma-paste borided in a gas mixture of 70 % H2 -30 % Ar using a mixture of 30 % SiC + 70 % B2O3 as a boron source. The samples were treated at temperatures of (700, 750 and 800)°C for (3, 5 and 7) h. The morphology of the formed boride layers was examined by light microscope and scanning electron microscope coupled to an EDS analyser. The borides present in the boride layer were identified by means of XRD analysis. The boron-activation energy for the AISI 316 steel was found to be equal to 250.8 kJ mol -1 . This value for the energy was compared to the literature data. A regression model based on a full factorial design was used to estimate the boride layers' thicknesses as a function of the boriding parameters: time and temperature. A comparison was made between the values of the boride layers' thicknesses estimated from the regression model with those given by an empirical relation. In addition, an iso-thickness diagram was plotted to predict the boride-layer thickness as a function of the processing parameters. This iso-thickness diagram can serve as a simple tool to select the optimum values for the boride layers' thicknesses for a practical utilisation in industry for this kind of steel. Keywords: plasma paste boriding, kinetics, borides, transition zone, activation energy, regression model V tem delu je bilo jeklo AISI 316 borirano s plazmo v me{anici plinov 70 % H2 -30 % Ar, z uporabo me{anice (30 % SiC + 70 % B2O3), kot vir bora. Vzorci so bili obdelani pri treh temperaturah (700, 750 in 800)°C v trajanju (3, 5 in 7) h. Morfologija nastale boridne plasti je bila preiskovana s svetlobnim mikroskopom in z vrsti~nim elektronskim mikroskopom, opremljenim z EDS analizatorjem. Boridi v borirani plasti so bili identificirani z rentgensko analizo. Aktivacijska energija bora v jeklu AISI 316 je bila 250,8 kJ mol -1 . Ta vrednost je bila primerjana s podatki iz literature. Za dolo~anje debeline borirane plasti v odvisnosti od parametrov boriranja (~as in temperatura), je bil uporabljen regresijski model, ki temelji na upo{tevanju faktorjev. Izvr{ena je bila primerjava debeline borirane plasti, dolo~ene z regresijskim modelom in primerjava s tisto, ki je bila dolo~ena empiri~no. Za napovedovanje debeline borirane plasti je bil postavljen diagram enake debeline v odvisnosti od procesnih parametrov. Diagram enake debeline je uporaben kot enostavno orodje pri izbiri optimalne debeline borirane plasti, za prakti~no uporabo v industriji za to vrsto jekla. Klju~ne besede: plazma boriranje s pasto, kinetika, boridi, prehodno podro~je, aktivacijska energija, regresijski model