Investigation of diffusion kinetics of plasma paste borided AISI 8620 steel using a mixture of B2O3 paste and B4C/SiC

In the present study, AISI 8620 steel was plasma paste borided by using various B2O3 paste mixture. The plasma paste boriding process was carried out in a dc plasma system at temperatures of 973, 1023 and 1073 K for 2, 5 and 7 h in a gas mixture of 70% H2 -30% Ar under a constant pressure of 10 mbar. The properties of the boride layer were evaluated by optical microscopy, X-ray diffraction, Vickers micro-hardness tester and the growth kinetics of the boride layers. X-ray diffraction analysis of boride layers on the surface of the steel revealed FeB and Fe2B phases. Depending on temperature and layer thickness, the activation energies of boron in steel were found to be 124.7 kJ/mol for 100% B2O3.National Boron Research Institute, Turkey as BOREN project numbered ‘2009-Ç0246’

Rapid Synthesis of Metallic Reinforced in Situ Intermetallic Composites in Ti-Al-Nb System via Resistive Sintering

Intermetallics are known as a group of materials that draws attention with their features such as ordered structure, high temperature resistance, high hardness and low density. In this paper, it is aimed to obtain intermetallic matrix composites and also to maintain some ductile Nb and Ti metallic phase by using 99.5% purity, 35-44 mu m particle size titanium, niobium and aluminium powders in one step via recently developed powder metallurgy processing technique - Electric current activated/assisted sintering system (ECAS). In this way, metallic reinforced intermetallic matrix composites were produced. Dominant phases of TiAl3 and NbAl3 which were the first compounds formed between peritectic reaction of solid titanium, niobium and molten aluminum in Ti-Al-Nb system during 10, 30 and 90 s for 2000 A current and 1.5-2.0 voltage were detected by XRD and SEM-EDS analysis. Hardness values of the test samples were measured by Vickers indentation technique and it was detected that the hardnesses of intermetallic phases as 411 HVN whereas ductile metallic phase as 120 HVN

Some properties of Cu-B4C composites manufactured by powder metallurgy

In this study, some properties of cold pressed Cu-B4C composites were investigated. Commercial copper powders with 40 µm particle size were reinforced with B4C in a 40µm particle size at ratios of 0, 1, 2, 3 wt.% for improving mechanical properties of copper used for electrical conductivity. Cu-B4C composites have been fabricated by powder metallurgy method and sintered at 700C for 2h in open atmosphere and then subjected to cold pressing following sintering process.  The presence of Cu and B4C which are dominant components in the sintered composites were confirmed by X-ray diffraction analyses technique and SEM-EDS.  Scanning electron microscope (SEM-EDS) was showed that B4C particles are distributed homogenously in the copper matrix. The relative densities of Cu and Cu-B4C composites sintered at 700°C are ranged from 97.5% to 90.19%. Microhardness of composites ranged from 80.65 to 87.5 HB and the electrical conductivity of composites changed between 90.04 %IACS and 68.87 %IACS. It was observed that cold pressed Cu - 1 wt.% B4C composites revealed promising physical properties

Characterization of transfer layers on steel surfaces sliding against diamond-like hydrocarbon films in dry nitrogen

Carbon-rich transfer layers on sliding contact surfaces play important roles in the tribological performance of diamond-like hydrocarbon (DLHC) films. In this study, we investigated the nature of these layers formed on M50 balls during sliding against DLHC films (1.5 mu m thick) prepared by ion-beam deposition. Long-duration sliding tests were performed with steel balls sliding against the DLHC coatings in dry nitrogen at room temperature, approximately 22 +/- 1 degrees C. Results indicated that the friction coefficients of test pairs were initially about 0.12 but decreased steadily with sliding distance to 0.02-0.03 and remained constant throughout the tests, which lasted for more than 250000 sliding cycles (approximately 30 km). This low-friction regime appeared to coincide with the formation of a carbon-rich transfer layer on the sliding surfaces of M50 balls. Micro-laser Raman spectroscopy and electron microscopy were used to elucidate the structure and chemistry of these transfer layers and to reveal their possible role in the wear and friction behavior of DLHC-coated surfaces

Tribological characteristics of boronized niobium for biojoint applications

Boride coatings on corrosion-resistant refractory metals are potentially used as implanting materials. In this research, we investigated wear mechanisms of boride coatings on pure niobium using a pin-on-disk tribometer in two different conditions i.e. in dry and using a simulated body fluid (SBF). Surface morphology studied using a scanning electron microscope (SEM) shows the compressed boride layer with indistinguishable regions such as coating intermediate transition layer and the substrate. The surface analysis after wear tests was conducted using an atomic force microscope (AFM). It was found that, in dry condition, the boride coating underwent deformation wear, and debris formed and accumulated at both ends of the track due to adhesion. In presence of SBF, the coating shows different mode of failure. The tribo-chemical wear dominates the wear mode. (c) 2006 Elsevier Ltd. All rights reserved

Formation of ultralow friction surface films on boron carbide

In this letter, we describe the formation and ultralow friction mechanisms of a surface film on boron carbide (B4C). This film results from sequential reactions between B4C and oxygen and between the resulting boron oxide (B2O3) and moisture; it can afford friction coefficients of 0.03 to 0.05 to sliding steel surfaces. At temperatures above 600 degrees C, B4C undergoes oxidation and forms a layer of boron oxide (B2O3) in the upper surface. During cooling to room temperature, the B2O3 reacts with moisture in the air to form a secondary him, boric acid (H3BO3) The sliding friction coefficient of 440C steel balls against this film is 0.04, compared to 0.7 against the bare B4C surfaces. Mechanistically, we propose that the ultralow friction behavior of the heat-treated B4C surface is due mainly to the layered-crystal structure of the H3BO3 film that forms on the sliding surface. (C) 1996 American Institute of Physics

Tribological characterization of smooth diamond films grown in Ar-C-60 and Ar-CH4 plasmas

In this paper, we describe the fundamental tribological mechanisms of smooth, nanocrystalline diamond films grown in Ar-C-60 and Ar-CH4 plasmas. These films can provide very low friction coefficients (approximate to 0.04 to 0.15) to sliding SiC and Si3N4 surfaces in air or dry N-2. To understand the mechanisms of friction and wear behavior in relation to the chemical and physical states of the diamond films, we used a variety of analytical techniques (electron diffraction, Raman spectroscopy, X-ray diffraction, scanning electron microscopy, atomic resolution transmission electron microscopy, near-edge X-ray absorption fine structure, and atomic force microscopy) both before and after sliding tests. The results of these studies verified that the films were made of sp(3)-bonded diamond nanocrystals (100-300 Angstrom) and that their surfaces were exceptionally smooth (i.e., 20-40 nm, root mean square). Additionally, pin-on-disk experiments verified that these films provided very low friction coefficients (0.04, in dry N-2) and wear rates, essentially comparable to those of natural diamond. Undoubtedly, such diamond films with smooth surface finish can have significant impact in a wide range of tribological applications

Multi-scale wear of a boride coating on tungsten

In this work, wear mechanisms of boride coating on pure tungsten using a pin-on-disk tribometer were investigated. The surface analysis after wear tests was conducted using an atomic force microscope (AFM). Research results showed that the boride coating underwent different types of wear modes: abrasion within a length scale from micrometer to millimeter and fracture within a few nanometers. Tungsten showed wear through plastic deformation and adhesion. These mixed wear modes could only be seen under the atomic force microscope. (c) 2005 Elsevier B.V. All rights reserved

Ultralow friction behavior of borided steel surfaces after flash annealing

In this letter, we describe the ultralow friction mechanism of borided steel surfaces subjected to a short-duration, or ''flash,'' annealing procedure. In this procedure, a borided steel surface is exposed to high temperature (600 to 800 degrees C) for a short time (3 to 5 min) and then cooled to room temperature in open air. During the high-temperature exposure, boron atoms within the borided layer diffuse to the surface and react spontaneously with oxygen in air. The reaction product is a thin boron oxide film. During cooling, the boron oxide reacts spontaneously with moisture in the surrounding air to form a thin boric acid film. The sliding friction coefficient of a Si3N4, ball against this flash-annealed surface is about 0.06, but is 0.5 and higher against the unborided or borided-only surfaces. Mechanistically, we propose that the ultralow friction behavior of the borided and flash-annealed surface is due mainly to the layered-crystal structure of the boric acid film that forms on the sliding surface. (C) 1996 American Institute of Physics

Characterization of borides formed on impurity-controlled chromium-based low alloy steels

This study reports on characterization of borides formed on impurity-controlled steels essentially containing 0.3% C, 0.20 P, 0.5% Cr; 0.3% C, 0.02% P, and 1.0% Cr. The formation of borides on the impurity-controlled chromium-based low alloy steel substrates was performed using thermochemical boronizing technique at 940 degrees C for 5-7 h. The boronizing molten salt bath, in which samples were immersed, consisted of borax, boric acid, and ferro-silicon. The presence of non-oxide ceramic borides (FeB, Fe2B, CrB, Cr2B) was confirmed by the classical metallographic technique, scanning electronmicroscopy and X-ray diffraction analysis. A wavelength dispersion X-ray dot map showed that boron diffused into the substrate, i.e, the diffusion layer is thicker than that of the coating layer. The hardness of non-oxide ceramic borides formed on the surface of steel. substrate were measured using a Vickers microhardness tester with a load of 2 N. The hardness of non-oxide ceramic borides was over 1500 HV. Depending on boronizing time, the thickness of borides ranged, from 25 to 200 mu m and the fracture toughness of non-oxide ceramic borides ranged between 3.42 and 4.40 MPa m(1/2). It was observed that chromium has a negative role on the fracture toughness and boride layer thickness. Metallographic analysis revealed that the borides formed on the surface of steel have a columnar structure. (C) 1999 Elsevier Science S.A. All rights reserved