Thixoforming : Semi-solid Metal Processing

270j 8 Tool Technologies for Forming of Semi-solid Metals
and coating constitution allowed the determination of a phase diagram for alumina as
a function of substrate temperature and electrical power injected into the plasma.
Special attention has been devoted to the mechanical properties of the coatings. Their
behaviour has been correlated with coating constitution and microstructure. The
latter can be porous. This porosity evolution can be understood by considering
different mechanisms, including the formation of residual chlorine bubbles in the
coatings. Finally, characterization of the adhesion, the tribological behaviour and the
thermal shock resistance has been performed in order to evaluate the performances
of the coatings in close to real conditions.
8.5.1
Experimental Procedure
Alumina thin films were deposited in a bipolar pulsed PECVD system. The plasma was
generatedbymeansofapulseswitchunit(SPIK1000A,Melec),whichwasconnectedto
aDCgenerator(MDX,AdvancedEnergy).Twostainless-steelelectrodeswithadiameter
of 160 mm were biased with a negative peak voltage (VP) ranging from 720 to 900 V.
A schematic drawing of the experimental setup is shown in Figure 8.26.
The pulse unit was operated in the bipolar symmetric mode with a rectangular
voltage pulse form. The negative and positive pulse times were adjusted to 50 and
30 ms, respectively, and the time between pulses was set to 5 ms (Figure 8.27).
The ion irradiation period of the growing film corresponds to the negative pulse,
which is followed by a positive period in which the film is under electron bombardment
and, consequently, the insulating layer is discharged. The discharge current and
voltage measurements were performed with a current probe (Tektronix TM P6003)
and a high-impedance voltage probe (Elditest GE 8115), respectively. The substrate
material was tempered hot working steel X38CrMoV5-1. Prior to deposition, the
Figure 8.26 Cross-sectional schematic view of the PECVD deposition chamber.