Microstructure performance of the ceramics-reinforced laser alloying composite coating on titanium

Li Peng

doi:10.1515/secm-2012-0082

Article Open Access

Microstructure performance of the ceramics-reinforced laser alloying composite coating on titanium

Li Peng

Published/Copyright: November 23, 2012

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From the journal Science and Engineering of Composite Materials Volume 20 Issue 2

Abstract

This work was based on the dry sliding wear of Al-Cu-Sn matrix composite coating deposited on titanium using laser alloying technique, the parameters of which were such as to provide almost crack-free coatings with minimum dilution and very low porosity. To our knowledge, it is the first time that Al-Cu-Sn+B₄C mixed powders were deposited as hard coating by laser alloying technique. Scanning electron microscopy results indicated that a laser alloying coating with metallurgical joint to the substrate was formed, and hard fine particles were produced in the matrix of the upper coating due to the in situ metallurgical reactions. Compared with titanium substrate, improvement of the microhardness and wear resistance were observed for such composite coating.

Keywords: lasers; microstructures; sliding wear; titanium

1 Introduction

Al-based alloys, which consist of Al-Sn, are widely used for sliding bearing applications due to their good load-carrying capacity, fatigue resistance, and wear resistance. In general, homogeneous and dispersed distribution of fine Sn in Al matrix is beneficial to friction and wear behavior [1].

Laser surface treatments have been widely used for improving the mechanical, chemical or tribological properties of the metal parts [2]. The surface alloying technique is based on a rapid thermal cycle of a thin surface layer by absorbing laser energy, which results in microstructural refinement, phase transformation, or the formation of alloyed layers. Recent years have seen the development of the TiB₂-TiC composites, which represent promising materials for use as wear resistance parts and high-temperature structural components [3]. Moreover, Cu played an important role in improving the surface performance of the laser alloying coating [4]. Recently, a laser surface alloying process has been used to modify the surface of these alloys by melting the alloying materials, such as Fe, Cu, Si, Co, Ni, etc., together with the substrate [5, 6], whereas less work has been carried out on the B-C-Al-Ti-Cu-Sn system, especially by laser alloying technique. Laser alloying of the Al-Cu-Sn-B₄C powders on titanium substrate can form a hard coating, which improved the wear resistance of the substrate. To our knowledge, it is the first time that Al-Cu-Sn+B₄C mixed powders were deposited as a hard coating by laser alloying technique. In this study, the microstructures and wear behavior of Al-Cu-Sn+B₄C laser alloying coating on titanium were discussed.

2 Experimental

A 2-kW continuous CO₂ laser was used to prepare the coating on the substrate. The materials used in this experiment were pure Ti samples (10 mm×10 mm×35 mm). The alloying surfaces were ground with emery paper to remove the oxide scale and rinsed with alcohol before laser alloying. A powder mixture of Al (≥99.5% purity, 50–150 μm), Cu (≥99.5% purity, 50–150 μm), Sn (≥99.5% purity, 50–150 μm), and B₄C (≥99.5% purity, 50–150 μm) was preplaced on the surface of the substrates with water glass to form a layer. Process parameters of laser alloying are as follows: laser power P=1.15 kW, scanning velocity V=6 mm/s, and laser beam diameter D=4.5 mm. Before the laser alloying process, the preplaced alloying powders were mixed until smooth, and a 0.8-mm layer of the preplaced alloying powders was smeared on the substrate with water glass as the binder. During the alloying process, argon gas at a pressure of 0.4 MPa was fed through a nozzle, which was coaxial with the laser beam. An overlap of 35% between successive tracks was selected. The compositions of the preplaced powders was 45 wt% Al+5 wt% Cu+15 wt% Sn+35 wt% B₄C.

Wear resistance of the laser alloying composite coating was tested by a WMM-W1 disc wear tester (Beijing Western Lofty Technology Limited Company, Beijing, P.R. China). The wear volume loss was measured after 40 min. The rotational speed of the wear tester was 465 rpm. The linear velocity of friction surface was 0.88 m/s. Microstructural morphologies of the composite coating were analyzed by means of A LEO 1525 scanning electron microscope (SEM) (Leo Germany Electron Limited Company, Ruhrgebiet, Germany), Philip Tecnai F 30G2 transmission electron microscopy (TEM) (FEI, Amsterdam, Holland), and energy dispersive spectrometer (EDS) (Leo Germany Electron Limited Company, Ruhrgebiet, Germany). An HV-1000 microsclerometer (Shanghai Zhongyan Instrument Manufacturing Plant, Shanghai, P.R. China) was used to test micro-hardness distribution of alloying coating.

3 Results and analysis

3.1 Microstructures

As can be seen, the laser alloying composite coating was metallurgically bonded to the titanium substrate in the sample (see Figure 1A). As shown in Figure 1B, the block/stick-shaped ceramics were dispersed uniformly in the matrix, which increased the wear resistance of the coating. During the alloying process, B₄C was dissolved due to the high temperature generated by the impingement of the laser beam, then delivered B and C to the melt pool. In addition, due to the uneven energy distribution of the laser melt pool, TiC in some locations absorbed enough energy from the laser beam and grew into dendrites (see Figure 1C). As shown in Figure 1D, fine white particles were produced in the middle coating. According to the EDS pattern and Table 1, C, Al, Ti, V, and Sn were present in the test location, indicating that the Al-Sn alloys and Ti-Al intermetallics were produced in the alloying coating. In fact, during the alloying process, when the temperature exceeded a certain value, a portion of Sn and Al melted due to the eutectic reaction and then smeared out along the boundaries of the matrix. During the eutectic reaction, the Al and Sn phases crystallized separately; the Al phase nucleated and grew from the existing Al matrix and then left the Sn phase along the boundaries of the matrix [7]. Ti-Al intermetallics also exhibited good properties, such as high stiffness and wear resistance, which was beneficial in improving wear resistance of the alloying coating [8].

Figure 1

SEM micrographs of the laser alloying composite coating. (A) The bonding zone, (B) alloying zone, (C) TiC, and (D) test location/EDS result.

Table 1

EDS result of the location in the alloying coating.

Location	Elements (wt%)
	C	Al	Ti	V	Sn
Test location	1.21	52.89	18.81	2.34	24.75

As shown in Figure 2A, fine particles were produced in the matrix of the upper coating. With the addition of Sn, the density of the melt pool increased; thus, Cu had a tendency to float onto the upper part of the melt pool. The EDS analysis results indicated that there were mainly Al, Ti, Cu, and Sn in this location (see Figure 2B). In fact, during the laser alloying process, Ti-Cu-Al alloy can be produced through in situ metallurgical reactions [9]. On the basis of the EDS result, it was speculated that the Sn-Al alloys were also produced in this location. Table 2 revealed that a few carbon atoms also existed as fine particles, which indicated that a small amount of the TiC phases were produced in this location. The test results indicated that the microhardness distribution of the fine-particle zone was in the range of 1400–1450 HV_0.2, which was higher than that of the location without fine particles (about 1300–1350 HV_0.2). Figure 2C and D show the microstructure of TiC and the selected area electron diffraction index calibration of the TiC [̅111] crystal zone axis.

$Figure 2 SEM micrographs of the composite coating. (A) Upper coating, (B) fine particles/EDS result, (C) TEM morphology of TiC, (D) electron diffraction pattern.$

Figure 2

SEM micrographs of the composite coating. (A) Upper coating, (B) fine particles/EDS result, (C) TEM morphology of TiC, (D) electron diffraction pattern.

Table 2

EDS result of the location in laser alloying coating.

Location	Elements (wt%)
	C	Al	Ti	Cu	Sn
Test location	0.89	51.78	23.87	11.78	11.68

3.2 Wear test results

Under the actions of the phase constituent and the fine grain strengthening of the titanium borides, the microhardness distribution of the laser alloying coating was in the range of 1300–1450 HV_0.2 (see Figure 3), which was approximately five times higher than that of the substrate (approx. 260 HV_0.2).

Figure 3

Microhardness distribution of the composite coating on titanium.

When the load was 49 N, the wear test result revealed that the wear volume loss of titanium substrate was five times higher than that of the alloying coating (see Figure 4). High wear resistance was primarily attributed to the characteristics of alloying coating in which its phase constituent showed excellent tribological properties and the fine grain strengthening of the titanium borides. In addition, due to the high microhardness, the wear resistance of the coating increased significantly. However, under the action of the pinning effect of the precipitates, the counterpart should overcome the hinder of these fine and dense precipitates during the wear process [10].

Figure 4

Wear volume losses of the composite coating and titanium.

The SEM images showed that the worn surface of the titanium substrate was very rough after 40 min wear time, and adhesion patches and deep plowing grooves were present in the worn surface of the substrate. As the microhardness of titanium was significantly lower than that of the counterpart, the hard asperities on the surface of the counterpart can easily penetrate into the sliding surface of the substrate, leading to the formation of deep grooves and adhesive features (see Figure 5A). According to the microstructure analysis results mentioned above, many hard phases, such as TiC, TiB₂, Ti-Cu-Al intermetallics, etc. were dispersed in the matrix of the coating, which prevented the stretching of wear grooves [10–12]. Furthermore, a fine microstructure of the alloying coating was obtained, which also showed excellent properties of plasticity and toughness, favoring the formation of the smooth wear surface (see Figure 5B). As mentioned previously, a large quantity of fine particles were produced in the upper coating, which included Ti-Cu-Al intermetallics and Al-Sn alloys. As shown in Figure 5C, the fine particles were present on the worn surface, which showed high microhardness, and prevented the formation of the adhesion patches and deep plowing grooves. Block-shaped ceramics asperities were also present in the worn surface (see Figure 5D), and the counterpart should overcome the hinders of the ceramics, so wear volume loss decreased.

Figure 5

Wear morphologies of (A) titanium, (B) composite coating, (C) fine particles, and (D) hard ceramic asperities.

4 Conclusions

Al-Cu-Sn matrix hard composite coating can be deposited on titanium by laser alloying technique. During the alloying process, when the temperature exceeded a certain value, a portion of Sn and Al melted due to the eutectic reaction and then smeared out along the boundaries of the matrix. The correct choice of laser alloying parameters provides Al-Cu-Sn+B₄C alloying composite coating on titanium with microhardness distribution in the range of 1300–1450 HV_0.2, which was approximately five times higher than that of substrate (approx. 260 HV_0.2). A laser alloying coating with metallurgical bonding to the substrate was formed, and hard fine particles were produced in the matrix of the upper coating under the action of in situ metallurgical reactions. Under the action of the phase constituent and fine grain strengthening, the wear volume loss of such composite coating was significantly less than that of titanium substrate.

Corresponding author: Li Peng, Mechanical and Electronic Engineering Department, Shandong Agricultural Administrators College, Jinan 250100, P.R. China

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Received: 2012-8-10

Accepted: 2012-10-23

Published Online: 2012-11-23

Published in Print: 2013-05-01

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Articles in the same Issue

https://doi.org/10.1515/secm-2012-0082

Keywords for this article

lasers; microstructures; sliding wear; titanium

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