Page 38 - PR 2014 2016 10 Materials and Nanotechnology

314 Materials and Nanotechnology | Progress Report

coatings with different periodicities (20, 10, 7.5 the HA, the YZ group was divided once again
and 4 nm) were produced. The coatings were (n=10): with and without mechanical cycling,
characterized by X-Ray Diffraction (XRD) and all ceramics were submitted to biaxial flexural
Transmission Electron Microscopy (TEM). The strength test (ISO 6872) and one way analysis
multilayer coating system was composed of al- of variance (ANOVA) (α=0.05) to compare the
ternate cubic rock-salt CrN and NbN layers, co- results. XRD results indicated that YTZP pre-
herently strained due to lattice mismatch. The sented a sigmoidal behavior due to the limited
film grew with columnar morphology through depth of XRD penetration (~6.3 μm). SEM and
the entire stratified structure. The periodici- OCT analysis confirmed a linear growth of the
ties adopted were maintained throughout the front as function of the time to all tempera-
entire coating. The 20 nm periodicity coating tures One way ANOVA showed a decrease in
showed separate NbN and CrN peaks in the mechanical strength (10-19%) after 140 hours
XRD patterns, while for the lower periodicity of HA (150°C) for all ceramics. Extrapolating
(≤10nm) coatings, just one intermediate lattice the depth of the transformed zone results to
(d-spacing) was detected. An almost linear 37°C, it was calculated that the front of the
increase of hardness with decreasing bilayer transformed zone would take around 1053
period indicates that interfacial effects can and 2104 years to reach 62.67μm and 85.55μm,
dominate the hardening mechanisms. depth that decreased the mechanical strength
of the YZ and LP, respectively. The association
Rietveld´s method of HA and mechanical cycling did not present

a deleterious effect for the YZ. The OCT was
The aim of this study was to evaluate the validated as a precise, easy and fast method
aging behavior of a dental Y-TZP submitted to evaluate the depth of the transformed zone
to an accelerated hydrothermal aging (HA). and for kinetics studies.
Specifically: a) determine the kinetic curve of
tetragonal to monoclinic phase transforma- Residual stress
tion (t-m); b) calculate the speed of the front
of phase transformation zone growth during Some effects caused by shot peening on Cr-Si-V
the HA; c) evaluate the relationship between steel have been studied, a process that aims
percentage of monoclinic phase and depth of to increase fatigue resistance. For this study,
phase transformation with the biaxial flexural he variation of parameters in the process are
strength (BF); d) evaluate the effect of the as- tools to better understand the mechanisms
sociation of HA and fatigue at the BF. Sintered that influence this property. The parameters
discs of dental Y-TZP (YZ- Vita YZ, LP- Lava used in this work were the variation of the grit
Plus) were submitted to HA at 120°C, 130°C and the pre-tensioning of the samples applied
and 150°C and analyzed by X-ray diffraction in leafs used in automotive springs of chrome
analysis (XRD). One specimen of each group silicon vanadium alloy steel (SAE 9254 + V).
was submitted to scanning electron microsco-
py (SEM) and optical coherence tomography The evaluation of the residual stress profile
(OCT) to calculate the phase transformation was performed by x-ray diffraction using the
depth and the speed of the front transforma- sen2Ψ method along the thickness in the
tion zone. Groups (YZ, LP and LA-Lava) were region where the stress is compressive. The
divided according to the aging time (n=10): 0, results show an anomalous effect in relation to
5, 25, 70 and 140 hours (150°C/3.01 bar). After the characteristic profile of the residual stress

Instituto de Pesquisas Energéticas e Nucleares