1. Introduction

   All moving parts in engineering machinery encounter wear to some extent. The use of wear-resistant materials is of fundamental importance for the industry because it leads to improve components life time and hence to reduce production costs in terms of maintenance problems, exchange of broken or unsafe parts etc… As a consequence, a major challenge of surface engineering is to find effective and low cost solutions for this purpose. This may be achieved on one hand by developing new high performance lubricants and on the other hand by using coating materials of high hardness, compactness and adhesion characteristics. An adequate choice of coatings must take into account the physical and chemical characteristics of the coating itself and the operating conditions (pressure, speed, temperature, environment…) and. The range of available coatings is becoming larger and as a consequence it is now possible to choose among a variety of surface treatments in relation to the exact needs of the end-users.

   In order to optimize the choice of the coating and/or surface treatment, it is necessary to determine the microstructural and mechanical properties of the whole part+coating system. The coating quality control is normally achieved in industrial practice by well known standard tests such as hardness, adhesion, traction and roughness. However, in many cases, a more detailed characterization at the microscopic level can be necessary. This can be obtained by means of spectroscopic techniques and by microscopic examination.

   The aim of the present paper is to give an overview of some ion beam techniques for surface material characterization and to introduce the "thin layer activation" (TLA). This technique is a complementary tool for wear properties investigation; It permits determining "in situ" the wear behavior of treated surfaces. It is not yet widely used in industry, but presents several advantages: it is safe, non destructive, easy to use and it can be applied on the "in service" conditions.

2. Characterization methods for surface material analysis

In recent years, together with the development of new coatings, progress has also taken place in the field of coatings characterization and testing. A large number of effective surface analysis techniques is now available. Among these, the most popular are electron and ion beam techniques which permit the microscopic characterization of properties such as crystalline structure, stoichiometry, trace element concentration and concentration profiles. All these parameters are of prime importance in materials science because they are directly related to wear resistance properties, for instance as related to the chemical origins of the mechanism of crack formation and chipping. Table 1 presents a non exhaustive list of some non-destructive techniques widely employed in surface science laboratories.

If material characterisation techniques are numerous, the ones for assessing directly the wear properties are basically limited to a few classic ones such as ultrasound, and some optical techniques. In the fiels of industrial coating, wear tests are commonly achieved by using the "pin of disc" system or methods based on coupled rotating cylinders. These methods are easy to implement but present the following drawbacks:

• they are poorly reproducible because of the large number of tribological parameters involved in the test.

• they do not reproduce the real operating conditions of moving parts.

• the information about the wear kinetics is limited : a precise weight loss measurement requires to interrupt the tests and remove the piece from the set-up; for instance it is not possible to access transient wear phenomena.

Although the previous techniques may be sufficient to satisfy the needs of coatings manufacturers, coating end-users sometimes require more specific wear and corrosion investigations directly related to their real needs. More than well defined physical parameters, they expect to be able to determine and optimize the life-time of the coated components under the real in-service conditions. The solution may be found by employing a recently developed technique: thin layer activation (TLA) ^1-7. This technique permits 'on line' monitoring of the wear process under real operating conditions (testing bench or industrial site). TLA has been already used successfully in the automotive, energy, and machinery industry. It is accurate, sensible, easy to implement and gives directly the wear kinetics. Its principle is based on the activation of the part under study by g -ray tracers. The part is subsequently inserted in its working environment and a g -ray detector is arranged close to it (1-50 cm). The measurement consists in monitoring the g -ray activity of the piece as a function of time. This activity can be directly related to the loss of matter of the piece during wear. An example of the use of TLA is further presented in order to illustrate its characteristics.

TLA has recently been employed in the energy industry; In nuclear power plants, stream generators pipes are subjected to wear and corrosion phenomena. The debris of removed matter enter in the primary fluid circuit. They are activated by the high neutron flux coming from the reactor and then are deposited along the circuit. As a consequence, activated debris create contamination areas responsible for a great part of the irradiation of maintenance workers. In order to reduce the irradiation doses, a complete study of the wear and corrosion phenomena in stream generators pipes has been performed. One of the challenges was to get precise information about the kinetics of loss of matter with the aim to further select, among different alloys, the more resistant ones. For this purpose, an experimental testing bench has been built in order to reproduce both the chemical and physical properties of the fluid circuit.

TLA first step: It consists in creating onto the near internal surface of the investigated part a thin g -ray activated layer. In our case, this has been achieved as shown in figure 1. A collimated particle beam passes through a sacrificial thin foil target. The beam creates g -ray tracers that escape the target and lodge in the pipe activated area. After a very low and safe implanted dose of radioactivity, the pipe is removed and arranged on its testing bench.

Figure 1 : Creation of a thin g -ray activated layer

TLA second step: on the pipe internal surface, the implanted tracers present a depth profile as shown in figure 2. For a relatively large depth (several tens of micrometers), the g -ray tracers concentration is a constant. The loss of matter due to wear/corrosion measurements is thus inversely proportional to the pipe remaining activity. A g -ray detector is monitoring the g -ray dose of the activated pipe (figure 3) and gives, by the help of figure 2, the wear kinetics as a function of time.

Figure 2 : Implanted tracers depth profile

Figure 3 : Wear measurements of stream generator pipes

TLA allows wear measurements of materials " on line " and "in situ". It also presents the following advantages:

• the sensitivity of loss of matter is high (< 1 m m).

• wear measurements are up to several hundred micrometers.

• TLA can be applied to almost all kinds of materials.

• the irradiation of the piece does not produce damaging effects or surface modification: TLA is non destructive.

• TLA is easy to use.

The application of ion beam advanced techniques to the industrial field of coatings is a relatively new kind of approach. These techniques are complementary to the standard mechanical characterization tests usually carried out in industrial laboratories. By associating material surface analysis techniques (table 1) and TLA, a complete study of wear can be achieved. In particular, the introduction of the relevant TLA technique will bring a new industrial tool to improve, through loss of material comparative measurements, the coatings wear resistant properties.

The author will be glad to provide further information and receive comments at: Olivier@lcfs.chim.unifi.it

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