Structure and properties of PVD coatings deposited on aluminium alloysby T Tański, K Lukaszkowicz*

Surface Engineering

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Structure and properties of PVD coatings deposited on aluminium alloys

T. Tan´ski and K. Lukaszkowicz*

Investigation results concerning microstructure and mechanical properties of the bilayer coatings are presented in this work. Gradient/monolithic coatings [Ti/Ti(C,N)-gradient/CrN, Cr/CrNgradient/CrN] were deposited onto the aluminium alloy (Al–Si–Cu) substrate by cathodic arc evaporation method. The microstructure of the achieved layers was examined using TEM and

SEM. Change of the chemical composition was measured using a glow discharge optical emission spectrometer. The physical vapour deposition coating structure is composed of small crystallites with an average size between 15 and 20 nm, depending on the coating type. Images (SEM) showed that the deposited coatings are characterised by compact structure without delamination or defects and that they closely adhere to each other. The Ti(C,N)/CrN coatings demonstrate the highest hardness and abrasive wear resistance.

Keywords: Aluminium alloys, PVD, Structure, TEM, GDOS, Properties

Introduction

A growing interest is noticed in light metals in recent years and especially in materials with low density and relatively high strength properties. This materials group includes, in particular, aluminium and its alloys. Aluminium alloys find their applications in the automobile and aviation industry. The big popularity of the aluminium alloys in these industry branches is connected with their general functional properties, namely, their low density of 2689 kg m23, good mechanical properties, good corrosion resistance and very good machinability. These properties are the reason for using them for car and aircraft engine bodies and housings of gearboxes, clutches, water pumps and rear axles, making it possible to decrease the operating expenses as well as decrease the fuel consumption. Owing to the limited fossil fuel stores and environmental problems associated with fuel emission products, there is a push in the automotive industry to make cars lighter in order to reduce fuel consumption. The use of aluminium alloys can significantly decrease the weight of automobiles without sacrificing structural strength.1–3

Deposition of hard coatings on material surface by physical vapour deposition (PVD) technology features one of the most intensely developed directions of improvement of the working properties of materials. This technology makes it possible to modify their surface by shaping their physical and chemical properties. Giving new operating characteristics to the commonly known non-ferrous alloys may be frequently obtained by depositing simple monolayer, multilayer, gradient or nanocomposite coatings using the PVD methods.4–10 Functional gradient coatings create a new class of coatings, with properties and structure changing gradually. Frequently, a rapid difference between the top coating and substrate properties occurs, causing a stress concentration in this area, both during the manufacturing and operation of the element. This causes fast degradation demonstrated by cracks and delamination of the coatings. The application of functional gradient coatings offers a possible solution of the issue.4–6,9,10 The aim of this paper is to examine the structure and mechanical properties of the gradient/monolithic coatings deposited by

PVD method onto the ACAlSi9Cu and ACAlSi9Cu4 casting aluminium alloys after heat treatment.

Experimental

The materials used for investigation were the aluminium alloys AlSi9Cu and AlSi9Cu4. The chemical compositions of the investigated aluminium alloys are presented in

Table 1. The computer aided engineering method was employed in this research for depositing the hard, wear resistant PVD coatings. The coating deposition process was carried out in the arc vacuum chamber based on the arc evaporation method, the so called cathodic arc evaporation, in anAr, N2 and C2H2 atmosphere. Cathodes containing pure metals (Cr, Ti) were used for deposition of the coatings. Just before the coating deposition process, the specimens were prepared, applying the standard procedure of chemical cleaning using the multistage washing in ultrasonic cleaner, and then they were ion etched in the chamber to clean the surfaces in the atomic scale and to activate it. Conditions of the coating deposition are presented in Table 2.

The examinations of thin foil microstructure and phase identification were made on the JEOL 3010CX TEM at an accelerating voltage of 300 kV. Microstructure and qualitative and quantitative chemical composition

Institute of Engineering Materials and Biomaterials, Silesian University of

Technology, Konarskiego St. 18a, Gliwice 44-100, Poland *Corresponding author, email krzysztof.lukaszkowicz@polsl.pl  2012 Institute of Materials, Minerals and Mining

Published by Maney on behalf of the Institute

Received 30 January 2012; accepted 9 June 2012 598 Surface Engineering 2012 VOL 28 NO 8 DOI 10.1179/1743294412Y.0000000033 analysis in micro-area investigation was performed using

SEMZeiss Supra 35 with the X-ray microanalysis (energy dispersive spectroscopy).

The cross-sectional atomic composition of the samples (coating and substrate) was obtained using a glow discharge optical spectrometer, GDOS-750 QDP, from

Leco Instruments. The following operation conditions of the spectrometer Grimm lamp were fixed during the tests: lamp inner diameter, 4 mm; lamp supply voltage, 700 V; lamp current, 20 mA; working pressure, 100 Pa.

Wear resistance investigations were performed using the ball on disc method. The tungsten carbide ball with a diameter of 3 mm was used as the counterpart. The tests were performed at room temperature by a defined time using the following test conditions: load, Fn55 N; rotation of the disc, 200 rev min21; wear radius, 2?5 mm; shift rate, 20?05 m s21.

The microhardness tests of the coatings were made with a Shimadzu DUH 202 ultramicrohardness tester.

Measurements were made with 10 mN load to eliminate the substrate influence on the coating hardness.