**1. Introduction**

174 Recent Trends in Processing and Degradation of Aluminium Alloys

Person, W.B. (1997). A Handbook of Lattice Spacing and Structures of Metals and Alloys,

Wierzbińska, M. & Sieniawski, J. (2010). Microstructural changes to AlCu6Ni1 alliy after

Paris-Braunschweig

pp.516−520

Vol.2, Pergamon Press, Oxford-London-Edinburgh-New York-Toronto-Sydney-

prolonged annealing at elevated temperature. *Journal of Microscopy*, Vol.237, No.3,

Aluminium is the third more abundant element in the Earth crust. The metal exhibits useful properties such as low density, high strength, good formability and a high resistance to corrosion. Aluminium can gain significant mechanical strength by means of alloying, whereby it is the most used metal after steel. In this sense, aluminium properties depend on its purity and its crystalline structure is face centred cubic [Wang et al, 1999].

Aluminium is, among other characteristics, malleable, easily machined and very ductile. Its high sensitivity to oxidation endows it with a waterproof passivation layer, typically 5-20μm thick according to the prevailing humidity, considerably adherent, which contributes to corrosion tolerance and general durability. The passivation layer consists of the amphoteric aluminium oxide Al2O3, often known as *alumina* or *aloxite* in mining and materials science. As corrosion is a major source of failure in Materials Engineering, aluminium is an obvious choice to face aggressive environments, including the atmospheric one.

Aluminium as a pure element has a low mechanical resistance which prevents its application under deformation and fracture conditions. Thus, low density combined with good resistance make aluminium alloys very attractive in design considerations. The properties of these alloys depend on a complex interaction among chemical composition, microstructural failures in solidification, thermal treatments, etc. although an increase in the alloy content tends, in general, to diminish the tolerance to corrosion. That is why quenching processes have been developed to improve the response to corrosion of highly alloyed materials. It is essential to select the precise alloy to match the resistance, ductibility, formability, solubility, corrosion tolerance, etc., required by an application.

Modifying aluminium composition by the adding nitrogen in an ion implantation process provides the treated samples with surface hardness and improved tribological properties by heating them in a nitrogen rich atmosphere. In this way, at low doses, aluminium nitride (AlN) becomes structured in the shape of clusters, the nitride content clearly increasing with the dose. Ion implantation is applied to pieces subjected to major friction and load forces such as rolling tracks, cylinder sleeves, etc., which require some core plasticity enabling

<sup>\*</sup> Raúl Valencia-Alvarado1, Arturo Eduardo Muñoz-Castro1, Rosendo Peña-Eguiluz1, Antonio Mercado-Cabrera1, Samuel R. Barocio1, Benjamín Gonzalo Rodríguez-Méndez1 and Anibal de la Piedad-Beneitez2 *1Instituto Nacional de Investigaciones Nucleares, Plasma Physics Laboratory A.P. 18-1027, 11801, México 2Instituto Tecnológico de Toluca A.P. 890, Toluca, México*

PIII for Aluminium Surface Modification 177

20 30 40 50 60 70 80 90

Angle (2θ) Relative intensity 38.47° 100 44.74° 47 65.13° 22 78.23° 24 82.43° 7

2θ [degrees]

**1** 

**1** 

**2 2** 

**2 2** 

**1** 

**1 Al 2 Al2 O3** **1** 

**1** 

Fig. 2. Reference simple spectrum

Intensity [a.u.]

Fig. 3. Reference simple diffractogramme

Table 2. Al peaks (JCPDS 4-0787 standard)

them to absorb vibrations and impacts, but maintaining their high surface hardness against wearing and deformation [Manova et al, 2001].

AlN was first synthesised in 1877. It is mostly formed by covalent bonds and exhibits a hexagonal crystalline structure which is isomorphic to the wurzite form of zinc sulphide. AlN is stable at very high temperatures in inert atmospheres. Its surface oxidation in air takes place at 700°C, although 5-10 nm layers developed at room temperature have been detected [Selvaduray and Sheet 1993]. This layer protects the material above 1370°C. AlN is stable in hydrogen and carbon dioxide atmospheres even at 980°C. It dissolves slowly in mineral acids, which attack its grain borders, and in strong alkalis that react with the grain itself. AlN is gradually hydrolysed but it is resistant to several fused salts such as chlorides and cryolites.

The experimentation discussed in the present chapter concerns the alloy composition of aluminium 6061 containing Mg and Si as shown in Table 1. With a view to evaluating the content per element and assessing the crystalline phases identified by DRX in each aluminium 6061 sample, micographs (figure 1) and corresponding spectra (figure 2) were obtained by SEM. The diffractogramme of the control (reference) sample, seen in figure 3, presents aluminium peaks at the 2*θ* values: 38.47º, 44.74º, 65.13°, 78.23° and 82.43° (JCPDS 4- 0787 standard) the last peak showing a greater intensity than that reported on Table 2. Aluminium oxide (Al2O3) can be detected at the 2*θ* angles 34.60°, 36.49°, 40.22° (JCPDS 12- 0539 standard) and at 42.76° (JCPDS 24-0493 standard).


Table 1. Al 6061 composition

Fig. 1. Reference simple micrograph

them to absorb vibrations and impacts, but maintaining their high surface hardness against

AlN was first synthesised in 1877. It is mostly formed by covalent bonds and exhibits a hexagonal crystalline structure which is isomorphic to the wurzite form of zinc sulphide. AlN is stable at very high temperatures in inert atmospheres. Its surface oxidation in air takes place at 700°C, although 5-10 nm layers developed at room temperature have been detected [Selvaduray and Sheet 1993]. This layer protects the material above 1370°C. AlN is stable in hydrogen and carbon dioxide atmospheres even at 980°C. It dissolves slowly in mineral acids, which attack its grain borders, and in strong alkalis that react with the grain itself. AlN is gradually hydrolysed but it is resistant to several fused salts such as chlorides

The experimentation discussed in the present chapter concerns the alloy composition of aluminium 6061 containing Mg and Si as shown in Table 1. With a view to evaluating the content per element and assessing the crystalline phases identified by DRX in each aluminium 6061 sample, micographs (figure 1) and corresponding spectra (figure 2) were obtained by SEM. The diffractogramme of the control (reference) sample, seen in figure 3, presents aluminium peaks at the 2*θ* values: 38.47º, 44.74º, 65.13°, 78.23° and 82.43° (JCPDS 4- 0787 standard) the last peak showing a greater intensity than that reported on Table 2. Aluminium oxide (Al2O3) can be detected at the 2*θ* angles 34.60°, 36.49°, 40.22° (JCPDS 12-

 %Si %Fe %Cu %Mn %Mg %Zn %Ti %Cr %other %Al 6061 0.4-0.8 0.7 0.15-0.40 0.15 0.8-1.2 0.25 0.15 0.04-0.35 0.15 Balance

wearing and deformation [Manova et al, 2001].

0539 standard) and at 42.76° (JCPDS 24-0493 standard).

and cryolites.

Table 1. Al 6061 composition

Fig. 1. Reference simple micrograph

Fig. 2. Reference simple spectrum

Fig. 3. Reference simple diffractogramme


Table 2. Al peaks (JCPDS 4-0787 standard)

PIII for Aluminium Surface Modification 179

This section contains a detailed description of the instrumentation used to carry out the PIII on the vacuum chamber with biased electrode, a specific high voltage modulator, and the

To accomplish the PIII process, the device was designed and constructed as shown schematically in figure 4. The plasma is produced in a stainless steel cylindrical vacuum chamber 0.6 m high, 0.3 m in diameter and 5 mm thick in the wall; thus the vacuum chamber volume is 0.042 m3. It has been provided with different ports for: a) pressure sensor, b) gas injection, c) electrode bias, d) target support, f) electrical probe diagnostics, g) spectroscopy diagnostics, along with other ports for future needs. The vacuum system

> Pressure sensor

> > Needle valve

Cathode SS304

Pulse high voltage

Gas inlet

**3. Instrumentation for PIII process** 

consists of a turbo pump with a 500 l/s capacity.

Electrical probe

falls into the 10-2 to 10-1 Torr interval.

**3.3 Pulse generator chamber** 

Main access

**3.1 Vacuum chamber** 

Anode SS316 L rod

**3.2 Plasma bias** 

diagnostic systems enabling to estimate the plasma parameters.

To vacuum turbo pump

Fig. 4. General view of the vacuum chamber discharge and its main accessories

The plasma is generated by a DC glow discharge between a stainless steel solid cylinder, acting as an anode, and the vacuum chamber as a cathode (figure 4). The DC power supply ranged within 0-1200 V/2A. The gas admission control to the vacuum chamber drives a gas dosing valve and the work gases being nitrogen and argon. The whole device is typically operated at a 1×10-6 Torr as base pressure and, during the PIII process, the work pressure

The pulse CD supply (see figure 5) consists of a three phase full wave rectifying circuit (D1- D6) coupled to a Variac which enables the user to select the CD output voltage level. Later

All in all, Aluminium and its alloys are attractive materials to the car, aviation, food and chemical industries as much as the pharmaceutical research. However, these materials lack surface hardness and other tribological qualities which limit their application nitriding is an effect time surface modification used to enhance corrosion tolerance in addition to wear resistance. In this chapter, a study of the formation of aluminium nitride (AIN) by means of the Plasma Implantation Ion Immersed (PIII) is presented.
