M.T. Vieira

In situ TEM study of grain growth in nanocrystalline copper thin films.

Nanocrystalline metals demonstrate a range of fascinating properties, including high levels of mechanical strength. However, as these materials are exposed to high temperatures, it is critical to determine the grain size evolution, as this process can drastically change the mechanical properties. In this work, nanocrystalline sputtered Cu thin films with 43 +/- 2 nm grain size were produced by dc-magnetron sputtering. Specimens were subsequently annealed in situ in a transmission electron microscope at 100, 300 and 500 degrees C. Not only was grain growth more evident at 500 degrees C but also the fraction of twins found. An analysis of grain growth kinetics revealed a time exponent of 3 and activation energy of 35 kJ mol(-1). This value is explained by the high energy stored in the form of dislocation, grain boundaries and twin boundaries existing in nanocrystalline copper, as well as the high probability for atoms to move across grains in nanocrystalline materials.

Influence of Ti addition on the properties of W–Ti–C/N sputtered films

Thin films of W–Ti–CyN were deposited by d.c. reactive magnetron sputtering from W–Ti targets with 0, 10, 20 and 30 wt.%Ti. The influence of titanium and interstitial element (carbon and nitrogen) contents on the structure, hardness and adhesion of the coatings was evaluated by X-ray diffraction analysis, ultramicroindentation and scratch-testing, respectively. The results show different compositional dependencies of the structure and grain size of the films. Hardness was related with the structure of the films, including lattice distortion and grain size. The higher hardness values (f50 GPa) were obtained for W–Ti–N films with 40–45 at.%N deposited from the W–20 wt.%Ti target in a reactive N atmosphere. However, these films present relatively 2 low adhesion to the substrates with critical loads of 30 N. The best compromise between hardness and adhesion was reached for W–Ti–N films with low nitrogen and titanium contents. 2003 Elsevier Science B.V. All rights reserved.

Determination of airborne nanoparticles from welding operations

The aim of this study is to assess the levels of airborne ultrafine particles emitted in welding processes (tungsten inert gas [TIG], metal active gas [MAG] of carbon steel, and friction stir welding [FSW] of aluminum) in terms of deposited area in pulmonary alveolar tract using a nanoparticle surface area monitor (NSAM) analyzer. The obtained results showed the dependence of process parameters on emitted ultrafine particles and demonstrated the presence of ultrafine particles compared to background levels. Data indicated that the process that resulted in the lowest levels of alveolar deposited surface area (ADSA) was FSW, followed by TIG and MAG. However, all tested processes resulted in significant concentrations of ultrafine particles being deposited in humans lungs of exposed workers.

Structure and chemical composition of W-C-(Co) sputtered films

Abstract W-C-(Co) films were deposited on high speed steel substrates by non-reactive r.f. and d.c. sputtering processes from sintered WC targets with 0,6 and 15 wt.% Co. The chemical composition of the coatings was determined by X-ray photoelectron spectroscopy and energy-dispersive X-ray spectrometry techniques and their structure was studied by X-ray diffraction. Negative substrate bias determines both the chemical composition and the structure of the coatings and its influence was studied systematically in the range from 0 to 400 V. The structure of the films ranges from amorphous to some forms of tungsten carbide: WC, β-WC1−x, α-W2C. The films orientation depends on the deposition procedure and substrate bias value.

Stainless steel coatings sputter-deposited on tungsten carbide powder particles

Abstract The aim of this work was to study the feasibility of a sputtering technique to coat WC powder particles, regarding it as an alternative to the conventional mixture of powders. For such purpose, a stainless steel 304 (AISI) coating was sputter deposited on WC powder particles using a magnetron sputtering equipment specially developed to coat powder particles. The morphology of the coated powder was characterized by scanning electron microscopy observations, Brunauer–Emmett–Teller and laser diffraction measurements. The crystallographic structure was determined by X-ray diffraction. Inductively coupled plasma–atomic emission spectrometer and electron microprobe analysis were used to characterize the amount, chemical composition and distribution of the sputtered coating. The characterization results indicated that all WC particles were coated and that all the steel constituent elements were deposited in the same original proportion. The coating had a ferrite b.c.c. structure and presented a columnar growth with some porosity. The compaction behavior of the coated powders was characterized by unidirectional pressing using pressures between 60 and 250 MPa. The maximum of relative density was attained for P ≥190 MPa, with values of 57–58% of relative density, comparable to that of non-coated powders, and without the need of any pressing binder to obtain green compacts resistant to handling. High sintered densities, of approximately 95%, were obtained at a relatively low temperature of 1325 °C with only ∼6 wt.% of binder phase in the coated powders.

Amorphous phase forming ability in(W–C)-based sputtered films

Abstract The effect of adding transition metals, Me, to the structure of (W–C)-based films obtained by sputtering has been studied by the means of electron probe microanalysis (EPMA), secondary ion mass spectrometry (SIMS), low angle X-ray diffraction (XRD), hot stage transmission electron microscopy (TEM), Mossbauer spectroscopy, extended X-ray absorption fine structure (EXAFS), magnetic measurements and differential thermal analysis (DTA). The results obtained for the films in the as-deposited conditions show two types of structures with different degrees of structural order. Films with Ti, Cr or Au are crystalline with a metastable structure of β -(W,Me)C 1- x with 1- x extending from near unity down to about 0.6. In opposition to these, films with Me=Group VIIIA transition metal, show crystalline→amorphous state transitions for Me percentages in the range 5–10 at.%. The structure of these films consists of small β -MC 1- x crystallites with a size of a few unity cells, surrounded by a disordered phase rich in element Me. Concerning the results obtained at increasing temperatures, the chemical and structural behaviour of the W–Me–C films depend on the affinity of carbon for the element Me. Strong or moderate carbide-forming elements (Ti or Cr) improve the stability of the crystalline phase at high temperatures; the films formed by W, C and a weak or non carbide-forming metal (Fe, Co, Ni, Pd or Au) change structurally in the temperature range studied.

The structure of thin films deposited from a sintered tungsten carbide with a high cobalt content (15 wt.

Abstract The microstructural and chemical composition characteristics of WCCo films deposited on high speed steel M2 (AISI) substrates by d.c. sputtering are discussed as functions of deposition rate, discharge power, discharge pressure, substrate bias and substrate temperature. Two different structures—α-W 2 C and amorphous—are found depending on deposition conditions. The α-W 2 C structure was only found for films deposited with high discharge pressures (19 Pa) and high negative substrate bias (greater than 200 V). The substrate bias has a fundamental role on the chemical composition of WCCo films, particularly on their cobalt content.

Comparison of deposited surface area of airborne ultrafine particles generated from two welding processes

This article describes work performed on the assessment of the levels of airborne ultrafine particles emitted in two welding processes metal-active gas (MAG) of carbon steel and friction-stir welding (FSW) of aluminium in terms of deposited area in alveolar tract of the lung using a nanoparticle surface area monitor analyser. The obtained results showed the dependence from process parameters on emitted ultrafine particles and clearly demonstrated the presence of ultrafine particles, when compared with background levels. The obtained results showed that the process that results on the lower levels of alveolar-deposited surface area is FSW, unlike MAG. Nevertheless, all the tested processes resulted in important doses of ultrafine particles that are to be deposited in the human lung of exposed workers.

Study of tungsten sputtered films with low nitrogen content

Abstract In this work, tungsten coatings doped with small percentages of nitrogen (% N at %) were deposited by sputtering and their characterization was focused in view of a possible future application as hard coatings. The films show an α-W (bcc) structure, significant changes in the X-ray diffraction peak positions not having been observed for the films containing nitrogen; however, these films have broader peaks than pure W films. All the films present a very dense cross-section morphology. Higher hardness values were found for the tungsten films containing nitrogen ( HV = 15 GPa → HV = [40–50 GPa ]). The scratch-test results show that the coatings fail cohesively, firstly by tensile cracking for loads in the range [30–40 N] and after by chipping for loads in the range [50–60 N]. Pure W films present only tensile cracking. The films are structurally stable up to 700°C annealing temperatures, even for a 12 h annealing time, only a narrowing of the diffraction peaks in relation to the as-deposited state being obtained. The hardness does not suffer significant alterations with increasing temperatures and annealing times.

The Influence of the Addition of a Third Element on the Structure and Mechanical Properties of Transition-Metal-Based Nanostructured Hard Films: Part I—Nitrides

Transition metal (TM) carbides and nitrides have been the most studied and investigated compounds since the beginning of the use of hard coatings to improve the performance of mechanical components. Since the pioneering study on the deposition and characterization of TiC and TiN, many different approaches have been followed in order to make these coatings perform better and better. In fact, the enthusiasm among researchers grew quite rapidly because the final results reached with coated components were so much better than with uncoated bulk materials. As a result, the application of hard coatings as a universal panacea for all the wear problems occurring in the mechanical industry was immediately installed. Nonetheless, it is obvious that whenever new situations were envisaged for the application of a hard coating, there were new demands that could not be satisfied with the existing Ti-based compounds. In most of the studies performed to develop “new” hard coatings, the supporting ideas were naturally based on the acquired knowledge of researchers