Carbon based coatings, diamond like carbon or amorphous hydrogenated carbon, which demonstrated excellent hardness, chemical inertness, low friction and wear resistance, have been successfully used as low load protective coatings for both magnetic and optical discs. These properties also make carbon based coatings promising candidates for the higher load wear protection of machine parts and tools.

Alternated a-C/a-C:Cr (about 2.5?m thickness) coatings were made by DC magnetron sputtering from graphite and Cr target in an argon discharge. Mechanical and tribological properties measured by indentation, scratch and pin-on-disc test.

The critical scratch load of a-C/a-C:Cr multilayer coatings for total failure is approach 100 N. The friction coefficient remains within the range of 0.08-0.1 at loads between 10 and 40 N during a pin-on-disc wear test, using 5 mm WC/Co ball as a slider. The wear depth only reaches 0.6 um A after a one hour wear test. The surface of a-C/a-C:Cr multilayer coatings undergoes cyclic deformation, and failure eventually occurs as a result of fatigue. The greater compliance and fracture toughness of the a-C/a-C:Cr multilayer coatings allows greater strains or strain energy to be stored before coating failure, and hence significantly improves wear resistance.

CVD and PVD coatings are today widely accepted in the tooling industry as wear protective coatings. The acceptance of diamond coated tools has increased in recent years in certain market segments, in the large market segments however a major breakthrough has not been achieved until now.

Two major problems in diamond deposition have been the reason for slower market growth than predicted. Large scale economic production and the adhesion problem. The first problem has been overcome today by some diamond deposition technologies, especially by the Balzers HCDCA (High Current DC-Arc)-technique. This deposition technology is today used successfully in production in different locations for the deposition of Balinit Diamond on carbide inserts and round tools.

Yet still a major drawback for succesful diamond tooling was the adhesion problem. While in the conventional PVD coating technology a broad range of substrate materials is suited for deposition, the requirements for diamond deposition on the substrate quality are much more restricted. Depending on the application different substrates and pretreatment technologies have to be applied. This requires a close relationship between the carbide-, tool manufacturer and the job coater.

The paper will report on the progress in combined optimisation of tool - and deposition technology to offer a solution for successful high end machining. About 10 years after the start of diamond deposition on cemented carbides Boehlerit offers now a new solution: In connection with the BAL (Boehlerit Aluminium) geometry and a special hard metal grade, developed for diamond deposition, these diamond coated indexable inserts are the long expected solution for cost effective machining of synthetic materials like glass fibre and carbon fibre composites: the lifetime can exceed by far that of PCD-tools.

Diamond coatings are currently used for a range of tooling applications due to their extreme hardness, chemical inertness and wear resistance. In this investigation an assessment is made of the erosive impact resistance of diamond coated cemented carbide. The diamond films were deposited using two techniques a microwave Surfajet and hot flame. The Surfajet is a type of surfatron in which the microwave power is coupled directly into the deposition chamber using a cylindrical discharge tube or antenna. The H₂, CH₄ and O₂ reactant gases are supplied coaxially with the microwaves (2.45 GHz) and enter the vacuum chamber as a plasma jet, from which the diamond film is deposited. Diamond film growth rates are approximately 0.5 µm / hour. In the hot flame technique the coatings are deposited from a C₂H₂ and O₂ gas mixture, film growth rates of up to 60 µm / hour are obtained.

In addition to the two deposition techniques, two different pre-treatment methods were used in order to enhance the adhesion of the diamond coatings on the cemented carbide surfaces. The first method involved the removal of the cobalt binder using acid leaching. The second pre-treatment involves neutralising the cobalt by reacting it with silicon at 1100^oC prior to diamond deposition. The resultant cobalt silicide layer that is formed on the surface of the cemented carbide does not cause graphitisation of the growing diamond film.

Erosive wear tests were carried out using a centripetal erosion tester in which SiC particles were impacted at a 90^o angle to the coated substrates. The impact momentum of the particles was increased until the diamond coating failed either due to film removal or spalling (hole formation). Once spalling was initiated coating delamination generally occurred over the entire area of exposure to the erosive particles. The erosive impact resistance of diamond was found to increase with film thickness in the range 1 to 16 µm. Surfajet deposited coatings survived higher impulse energies than the diamond films deposited by the hot flame technique. For example a 6.4 µm Surfajet film failed at an impulse to penetration of 5.0 µmNs, compared to 1.5 µmNs for a hot flame coating with similar thickness. Other factors found to affect erosive impact performance include diamond film quality, adhesion, roughness, porosity and crystallite size.