T. Bacci

Dry sliding wear mechanisms of the Ti6Al4V alloy

Abstract The dry sliding behaviour of the Ti6Al4V alloy was studied in order to highlight the mechanisms responsible for the poor wear resistance in different load and sliding speed conditions. By increasing the sliding speed, a transition from oxidative wear to delamination occurs with a corresponding minimum in the wear rate. This minimum shifts towards lower speeds by increasing the load. The results were supported by the analysis of the wear debris and of the worn specimens, and discussed on the basis of the main mechanical and chemical characteristics of Ti alloys which influence the wear resistance.

Glow-discharge nitriding of sintered stainless steels

Abstract The glow-discharge nitriding process is particularly suitable to harden the surface of sintered stainless steels components, owing to the high porosity levels of these materials. On wrought austenitic stainless steels this treatment produces a metastable phase, known as supersaturated austenite or S phase, which has shown high hardness values and good corrosion resistance. In the present paper the influence of glow-discharge nitriding process on the microstructural and mechanical properties of AISI 316L austenitic sintered stainless steel has been evaluated and it is compared with the results obtained with ion-nitrided martensitic (AISI 410) and ferritic (AISI 430L) sintered stainless steels. The ion-nitriding treatment, performed at 773 K for 4 and 8 h, produces modified surface layers. The microhardness profiles show high hardness values in the modified layers and a steep decrease to matrix values, thinner hardened layers and lower hardness values are observed on AISI 316L samples, in comparison with AISI 410 and AISI 430L samples. The S phase is detected on the modified layers of the ion-nitrided AISI 316L samples. The crystallographic characterisation has shown that a face centred tetragonal lattice as base for this phase fits well the diffraction spectra, in respect of the ‘traditional’ face centred cubic lattice usually adopted, since the lattice shows a strong distortion, in spite of this, the d -spacing values calculated with a f.c.t. base show a good agreement with literature data, when the used f.c.c. indexing is modified for the f.c.t. lattice.

Effects of load and sliding speed on the tribological behaviour of Ti6Al4V plasma nitrided different temperatures

Abstract Dry sliding tests were carried out under different load and sliding speed conditions on Tiue5f86Alue5f84V alloy plasma nitrided at three temperatures 973, 1073 and 1173 K. The results were interpreted on the basis of the evolution of the frictioc basis of the evolution of the friction coefficient and characterizing the wear debris and worn surfaces in order to understand the acting wear mechanisms. A comparison was also made with the results obtained with untreated specimens, tested under the same conditions. Plasma nitriding can noticeably improve the dry sliding resistance of the Tiue5f86Alue5f84V alloy. The nitriding temperature must be chosen according to the main wear mechanism observed under specific load and sliding speed conditions. When wear is determined by the resistance of the compound layer (low loads and low sliding speeds), the nitriding treatment has to be carried out at 1073 K. In this case, the compound layer has optimal properties with respect to resistance to adhesion and fragmentation. When the material is exposed to delamination (high loads and high sliding speeds), the strength of the diffusion layer has to be maximized. In this case, the nitriding temperature should be as high as possible (1173 K in the present investigation) in order to enhance the hardening of the diffusion layer.

Glow-discharge nitriding and post-oxidising treatments of AISI H11 steel

Abstract Post-oxidising treatments can be performed to improve the corrosion resistance of nitrided steel components. The glow-discharge process appears particularly attractive, since it allows to carry out both nitriding and post-oxidising treatments in a single technological operation, simply varying the treatment atmosphere and the working conditions. In the present paper the effects of using air as the treatment atmosphere of glow-discharge post-oxidising treatment have been evaluated. Nitriding treatments were performed at 773 K for 5 h, and post-oxidising treatments were carried out at 623 and 773 K for 1 h. The nitriding and nitriding+post-oxidising treatments produce modified surface layers consisting of an outer compound layer and an inner diffusion layer. When the post-oxidising treatments are performed, the compound layer consists of an oxide layer on the top of the nitride layer. With a 623 K post-oxidising treatment the oxide layer consists mainly of magnetite, Fe 3 O 4 , while when post-oxidising is carried out at 773 K it consists essentially of hematite, Fe 2 O 3 . In the inner nitride layer iron (γ′-Fe 4 N, e-Fe 2–3 N) and chromium (CrN) nitrides are present. In all the treated samples high hardness values are obtained in the modified layers. The hardness profiles of the post-oxidised samples are only slightly affected by the 773 K post-oxidising treatment. The corrosion resistance of the treated samples was tested and the results show that the nitriding+623 K post-oxidising treatment is able to significantly improve the corrosion resistance of the untreated and treated AISI H11 samples.

Reactive plasma spraying of titanium in nitrogen containing plasma gas

Abstract The low hardness and the poor tribologic characteristics of titanium can be positively modified by means of reactive plasma spraying. The present study describes experimental results concerning reactive low pressure plasma spraying of pure titanium powders in an Ar/N 2 plasma gas in order to obtain thick titanium composite coatings with a high percentage of hard titanium nitrides (TiN and Ti 2 N). As measured by means of X-ray diffraction, the volume fraction of nitrides in the coating with increases increasing nitrogen content in the plasma gas mixture and with increased spraying distance. The nitrogen content of the coating shows a linear relationship with the microhardness of the coating. By varying the Ar/N 2 plasma gas ratio, adherent hard composite coatings, up to Knoop 1200 hardness and with low porosity are obtained. Such characteristics make them particularly suitable in all those technological applications where good corrosion resistance of titanium and high tribologic properties are required.

Air treatment of pure titanium by furnace and glow-discharge processes

Abstract The high oxygen affinity of titanium can be used to improve the surface hardness and wear resistance of titanium and titanium alloy components by means of thermal treatments. In the present paper a comparison of the effects of glow-discharge and furnace processes, carried out using air as treatment atmosphere, on the microstructure and mechanical properties of commercially pure titanium samples is reported. Both treatment types, performed at 973 and 1173 K for 0.5, 2 and 4 h, produce modified surface layers: the outer layer (compound layer) consists essentially of TiO 2 and small amounts of TiN x O y , while the inner layer (diffusion layer) consists of α-Ti crystals rich in interstitial atoms. The plasma treatment is shown to be more efficient in hardening titanium surface layers than the furnace process: by using the same treatment temperature and time, plasma treated samples have a thicker hardened layer, with higher hardness values than the furnace treated ones. A crystallographic characterisation of the α-Ti solid solution has also been performed and the a , c lattice parameters have been evaluated. Plasma treated samples show larger lattice parameters and c / a values than the furnace treated samples, suggesting a higher concentration of interstitial atoms due to the plasma treatment.

Wear resistance of Ti–6Al–4V alloy treated by means of glow-discharge and furnace treatments

Abstract The contemporary glow-discharge oxidising and nitriding treatment has shown to produce modified surface layers with enhanced hardness properties, in comparison with the conventional ion-nitriding process. The aim of the present study was to evaluate the tribological properties of ion-oxinitrided samples and to compare their wear behaviour with the one of furnace oxinitrided and ion-nitrided samples. At low coupling loads (50 N) the wear volumes of the treated samples result small and comparable for all the tested velocities. On the other hand, when high coupling loads are used (100 N), the wear of the ion-nitrided samples is higher than that of the oxinitrided ones, this effect becoming more remarkable as sliding velocity increases; moreover, the ion-oxinitriding treatment allows to achieve lower wear volumes than the ones obtained by means of the furnace oxinitriding process.

Influence of the treatment atmosphere on the characteristics of glow-discharge treated sintered stainless steels

Abstract The surface hardening of sintered stainless steel components is a major goal, in order to extend the applications of these materials. The aim of the present study was to evaluate the influence of the treatment atmosphere on the microstructure and mechanical properties of AISI 316L austenitic and AISI 410 martensitic sintered stainless steels by performing glow-discharge nitriding and nitrocarburizing treatments. The treatments, performed at 773 K for 8 h, produced modified surface layers. The use of a nitrocarburizing atmosphere is able to promote the formation of e carbonitride [Fe 2−3 (N, C)], especially on AISI 410 martensitic samples. The microhardness profiles of the treated samples show high hardness values in the modified layers and a steep decrease to matrix values. The AISI 316L samples have thinner hardened layers and lower hardness values, in comparison with AISI 410 ones. Thinner hardened layers with a steeper decrease to matrix values are obtained using a nitrocarburizing atmosphere, if compared with those of nitrided samples. On the modified layers of both the nitrided and nitrocarburized AISI 316L samples the metastable S phase is detected. The X-ray diffraction analysis has shown that its lattice parameter values, evaluated by fitting it with a face centred tetragonal lattice, are influenced by the treatment atmosphere and that the amount of this phase increases when a nitrocarburizing atmosphere is used.

Plasma nitriding and nitrocarburising of sintered FeCrMo and FeCrMoC alloys

Abstract The effect of plasma nitriding and nitrocarburising on the microstructure, and the microhardness and the wear resistance of two sintered alloys, Feue5f8Crue5f8Mo and Feue5f8Crue5f8Moue5f8C, was studied. Nitrocarburising improves the microhardness profile and the wear resistance of the C-free material with respect to nitriding, while it does not induce any appreciable improvement with respect to the nitrided C-alloyed material. The wear resistance of the nitrocarburised C-free material is the same as that of the nitrided C-alloyed material. Nitrocarburising tends to induce the grain boundary precipitation of carbonitrides in the diffusion layer.

Improvement of corrosion resistance of austenitic stainless steels by means of glow-discharge nitriding

Abstract Austenitic stainless steels are employed in many industrial fields due to their very good resistance to general corrosion in several environments. However, their use is limited due to the fact that they suffer localised corrosion in specific environments, particularly in chloride-ion rich solutions. Low temperature nitriding treatments can improve both corrosion resistance in chloride-ion containing media and surface hardness, due to the formation of a metastable phase known as expanded austenite or S phase; this phase can be outlined as a supersaturated interstitial solid solution of nitrogen in the expanded and distorted γ-Fe f.c.c. lattice. We review the main experimental results obtained in our research on low temperature glow-discharge nitriding of austenitic stainless steels. By means of proper treatment parameters, low temperature glow-discharge nitriding is able to markedly improve the corrosion resistance of austenitic stainless steels, such as AISI 316L and AISI 202 in chloride-ion rich solutions in comparison with the untreated alloys.