Optical evaluation of the surfaces of the CuAlBe samples after the test of resistance to harsh wear stresses in an explosive atmosphere

. A material with both non-sparking and good mechanical properties based on CuBe-Al elements was proposed for anti-sparking materials intended for use in potentially explosive atmospheres (design and manufacture of technical equipment, tools and implements intended for use in potentially explosive atmospheres). The alloy was obtained using a classical induction furnace from Cu-Be master alloy and pure Al. Using hot-rolled bands (heated to 800 °C and rolled from ingots with 10 mm diameter to 2 mm thickness plates) experimental devices were made to perform laboratory tests in explosive mixtures for the acceptance of spark arresting materials intended for use in potentially explosive atmospheres. A hard wear process by forcefully applying some steel elements to the experimental CuAlBe alloy was carried out in an enclosure with an explosive atmosphere consisting of an atmosphere with 10% H2. The main purpose of the tests is to verify whether or not the tested materials produce mechanical sparks. They are considered non-sparking materials and are checked/tested accordingly. Their appearance was not observed in the first 16000 friction tests between the two materials. The tests left deep traces of wear on the surface of the experimental alloys, leading to damage to their surface. The wear marks were analyzed macroscopically by optical microscopy (LOM). The test response after 16 000 frictions was negative to explosion and the material can be used as non spark alloy


Introduction
Protection against explosions is taken into consideration more and more as much conflagrations put in danger the existence and integrity of the employees based on the accidental consequences of the fires and pressure, the possibility of dangerous process compounds and due to the usage of O2 from the surrounding atmosphere breathed by the workers.A deflagration happen when a carburant type gaseous mixed with air (a minimum quantity of O2) get to the expin an electric furnace losive limits in the presence of an ignition source.
Explosion-proof equipment recommends the use of robust materials and proactive construction to prevent unwanted ignition of flammable substances in hazardous locations.Unlike regular work equipment or electronics, these state-of-the-art units are designed to mitigate sparking by using special non-ferrous materials [1][2][3][4].
The usage of explosion -resistance equipments with non-sparking components (more on this later) can be found in OSHA's 1910 guidelines.146Appendix D: Confined Space Pre-Entry Checklist, which suggests that power tools used in confined spaces they must not produce sparks [5].
Electrical sparks are a primary source of ignition in many cases of equipment and tools around industrial facilities.To reduce the creation of sparks, explosion-proof equipment is usually constructed of non-sparking materials.With this in mind, most non-ferrous metals used for explosion-proof systems have a high thermal conductivity [6,7].
Non-metallic, non-sparking materials are also commonly used in explosion-proof equipment [8,9].These include plastics, wood, thermoplastic polymers, leather, etc.Such materials are mainly used for the manufacture of non-sparking tools -hammers, shovels and gloves [10,11].For explosion-proof lighting systems, strong, non-sparking polymers such as polycarbonate or carbon fiber can be used to prevent sparking.
In this paper the authors present the first experiments on CuAlBe alloy as non-spark materials through standardized wear tests.

Experimental details
Using Cu-Be master alloy (4%wtBe) and high purity Al we obtain a CuAlBe (~10wt%Al and ~2 wt%Be) in an electric furnace at 1150 ºC using a ceramic crucible, see Figure 1a.Homogeneous ingots, no surface defects were observed, with 10 mm diameter and 100 mm length were obtained in a metal casting form.CuAlBe ingots were heat treated at 800 ºC for ten minutes and hot rolled using a rolling mill, see Figure 1b.The degree of reduction was ~10% of the initial diameter until plates with length of 16-18 mm and thickness of 2-3 mm were obtained.Each reduction step was first accompanied by heating the material to 800 ºC.The resistance to frictions experiments, in order not to produce hot sparks that can ignite an explosive atmosphere, were realized at National Institute for Research and Development in Mine Safety and Protection to Explosion -INSEMEX based on impact and frictions tests standard.The experimental materials were prepared and attached to the steel plates, see Figure 2a, given a central area for the wear in explosive atmosphere.The experiment was realized with steel wearing elements, see Figure 2a, and the contact, repeated for 16000 times was between the steel element and CuAlBe plates.For impact tests, round steel balls were prepared and CuAlBe plates were fixed on the exterior part, see Figure 2b, in order to be exposed to the impact shock with a steel plate.

a b
Fig. 2. Elements prepared for frictions, a and impact tests, b in an explosive atmosphere.
The plates for impact test were hot deformed straight on the steel ball and mechanically attached on the ball.The exposed area will get in contact with a massive steel plate in an explosive atmosphere in order to observe the hot sparks formation and their capability (or not) to ignite an explosive atmosphere.

Experimental results
Starting from the fact that bronzes with Be are intended for the manufacture of hard nonsparking tools (hammers, chisels, pliers, etc.) used in refineries, in the chemical industry and in mines, special bearings, parts subject to wear, gears, cams, stainless spiral springs, leaf springs subjected to wear, parts for antimagnetic watches, bellows and diaphragms in measuring instruments and automation, pressure welding electrodes (these also contain Co), electrical contacts and spring blades in electrical circuits.The authors propose an alloy Cu-Al-Be as a suitable solution for making elements, for example gears, which work in the explosive atmosphere and also perform a mechanical work during operation.
After the experimental tests (harsh wear to follow the sparks production during the metals contact) the plates surface presents a removal of the oxides layer from the surface, figure 3a and b.The most affected plates were those from the center were the steel counterpart action was more intense, see Figure 3b.No wear stains were observed on the steel support of the plates meaning that the entire contact area was realized between CuAlBe plates and the steel wear elements, see Figure 3a and b.In the central placed plates case, see Figure 3b, large cracks can be observed on the wear area.
Softer material, CuAlBe, is observed on the counterpart steel elements, see Figure 3c  and d, confirming the harsh aspect of the wear test that removes the material from the plates.No harder condition can be considered in the function of a CuAlBe element in explosive atmosphere.In Figure 4 an optical image of the worn area is presented.Material overlaps due to hard material wear may be observed.No cracks or pores were identified in this area.The sparks resulting from the friction process are considered harmless (the tested pair of materials passed the test or not) if: -there is no ignition during the first 16,000 frictions in the explosive mixture; -no more than 8 ignitions occur during the next 16,000 frictions in the explosive mixture enriched with oxygen up to 25%; In conclusion, considering the test results presented above as well as the test conditions, the tested materials (samples F1) are considered non-sparking materials.

Conclusions
Experimental results analysis reveals the follow conclusions: -CuAlBe is a material suitable for non-spark metallic elements and after 16000 times of friction tests with a steel element no ignition of the explosive atmosphere was initiated; -The technology of obtaining CuAlBe plates was developed in the laboratory from melting (resistance furnace), heat treatment and hot-rolling without inconveniences.

Fig. 3 .
Fig. 3. Worn plates after the resistance wear test, a first set of CuAlBe plates, b second set of CuAlBe plates, c and d steel elements used for the forced wear of CuAlBe plates.

Fig. 4 .
Fig. 4. Optical image of the worn surface.The main conclusions of the wear tests in an explosive atmosphere were summarized in table 1 by the specialists from National Institute for Research and Development in Mine Safety and Protection to Explosion -INSEMEX.

Table 1 .
Results after the friction test in an explosive atmosphere.