Teodoro Valente

Plasma spray deposition and high temperature characterization of ZrB2–SiC protective coatings

Abstract Refractory metal borides are the object of special interest for aerospace applications requiring properties of chemical and mechanical resistance in ultra high temperature, such as nose and leading edges of re-entry space vehicles. The main objective of the research is the fabrication and characterization of plasma sprayed zirconium diboride–silicon carbide composite coatings and free-standing components for high temperature applications. High and low pressure plasma spray in a controlled atmosphere were selected as manufacturing techniques for the deposition of ceramic coatings. Fine ZrB 2 and SiC precursors were agglomerated and preconsolidated into spherical, hollow powders for better flowability and silicon carbide thermal protection during the interaction with the plasma. Coatings and free-standing tubular specimens were fabricated and tested for high temperature behaviour. Thermogravimetric analysis, surface morphology investigation and high temperature X-ray diffraction showed that the addition of approximately 25% SiC induces a mechanism of self-protection of the ceramic material during heat treatment in oxidizing environments up to approximately 2100 K.

Parametric study of an HVOF process for the deposition of nanostructured WC-Co coatings

Nanocrystalline WC-Co coatings were deposited by high velocity oxyfuel from commercial nanostructured composite powders. Processing parameters were optimized for maximal retention of the nanocrystalline size and for minimal decarburation of the ceramic reinforcement. Thermochemical and gas-dynamical properties of gas and particle flows within the combustion flame were identified in various operating conditions by computational fluid-dynamics (CFD) simulation. Significant improvements of the mechanical properties of the coatings were obtained: a decrease of the friction coefficient was measured for the nanostructured coatings, together with an increase of microhardness and fracture toughness.

Statistical evaluation of Vicker's indentation test results for thermally sprayed materials

In this experimental investigation, Vickers microhardness tests were used to assess the heterogeneity of a wide range of thermally sprayed coatings. Three different loads were used to evaluate the influence of the test volume on load-hardness dependence. Statistical elaboration of obtained results were performed by using variance analysis, Students and Fishers tests, Gaussian and Weibull distributions, Meyers theory, in order to separate variation due to scatter of data and due to different materials properties. A load-hardness dependence was evidenced especially for ceramic coatings, due to their brittle nature, whereas for the two metallic coatings tested (Ti-6Al-4V and CoNiCrAlY) a bimodal distribution seems to exist at small loads.

Corrosion resistance properties of reactive plasma-sprayed titanium composite coatings

Abstract Among thermal spraying methods, an attractive technical possibility lies in the fabrication of protective coatings or free-standing components by means of reactive plasma spraying (RPS) techniques. Using reactive gases, such as nitrogen or methane, it is possible to synthesize hard nitride or carbide phases in reactive metals like Ti, Cr or Al. In this investigation composite titanium-nitrides/titanium coatings produced by RPS through a controlled atmosphere plasma spray system (CAPS), were electrochemically tested to evaluate their corrosion behaviour. Two environments were selected: a neutral (0.5 M NaCl) and an acid aqueous solution (0.5 M NaCl+1 M HCl). The influence of porosity and nitrogen content on the corrosion resistance has been investigated. Polarization curves of coated samples, detached coatings, AISI304 substrate and commercially pure titanium (grade 2), are also reported and discussed. The corrosion resistance of coated samples was found to be mainly dependent on porosity values, thus optimization of plasma spraying parameters assumes a fundamental role to obtain wear and corrosion resistant deposits.

Part I: characterization of Cr3C2-25% NiCr reactive plasma sprayed coatings produced at different pressures

The present work was performed with the aim of characterizing various plasma sprayed Cr 3 C 2 -NiCr coatings produced by using different processing pressures between 300 and 1200 mbars, in a nitrogen controlled atmosphere CAPS system. X-Ray diffraction was carried out on all coatings by using Bragg-Brentano geometry. The phases identified in the as-supplied powder Cr 3 C 2 /Ni-Cr were fcc Ni-Cr and orthorhombic Cr 3 C 2 . In contrast to the original powder, the coatings showed evidence of Cr 3 C 2 , Ni-Cr and either Cr 6.2 C 3.5 N 0.3 or Cr 3 C 1.52 N 0.48 carbo-nitride phases depending on the ratio of C/N in the coating. The presence of Cr 7 C 3 and CrO 3 was also identified in the coatings deposited at atmospheric pressure. The volume fraction of carbide plus nitride phase in the coating was always less than the volume fractions of the carbide phase in the original feed stock powder. The volume fraction of carbide plus nitride phase was found to depend quite markedly on the spraying parameters such as pressure, power input, spraying distance and substrate cooling. The results have been presented in terms of spraying efficiency, ξ. The highest value of ξ (86.4%) was obtained for coatings produced at 1200 mbar pressure, a spraying distance of 120 mm, without N 2 cooling and the lowest value (25.2%) was found for the coatings deposited at 300 mbar for the same spraying distance but with N 2 cooling. The presence of graphitic carbon was detected in all samples especially for specimens where the fraction of carbide plus carbo-nitride phases was the greatest. The influence of spraying parameters such as pressure, power input, spraying distance and substrate cooling on the microstructure was determined. The highest microhardness value of 2296 HV 50 was found for the carbide plus carbo-nitride phase contained in the specimens produced at 1200 mbar spraying pressure, 120-mm spraying distance and substrate heating at 600°C.

Part II: tribological performance of Cr3C2-25% NiCr reactive plasma sprayed coatings deposited at different pressures

The present work has been undertaken to determine the sliding wear behavior of plasma sprayed coatings deposited using different pressure conditions, ranging from 300–1200 mbars, in a nitrogen atmosphere using a controlled atmosphere plasma spray system (CAPS). The spraying distance and the substrate temperature during deposition were also varied. Sliding wear tests were performed against WC-6% Co balls using a pin-on-disc tribometer. The highest sliding wear resistance was obtained for the samples processed at a pressure of 1200 mbar, a spraying distance of 120 mm and substrate heating to 600°C, for which an average value for the specific wear coefficient, k, of 5.72 E-07 mm3/N m was determined. An increase in the spraying distance from 120 to 140 mm, keeping all the other parameters constant produced an increase in the wear volume of 33%. However, for the same spraying pressure and distance, cooling the substrate resulted in an increase in the wear volume of nearly 1125% when compared to the coating produced with optimum conditions. In addition, a decrease in wear resistance of 83% occurred when the pressure was varied between 1200 to 300 mbar, which was attributed to a decrease in the carbide deposition efficiency, ξ, defined as the relationship between the (Cr3C2) coating/(Cr3C2) powder. This new material, consisting of chromium carbide, Ni–Cr, carbo-nitride phase and graphitic carbon had excellent sliding wear resistance that was approximately 23 times better than that reported for conventional HVOF Cr3C2–25NiCr coatings.

Implementation and Development of the Incremental Hole Drilling Method for the Measurement of Residual Stress in Thermal Spray Coatings

The experimental measurement of residual stresses originating within thick coatings deposited by thermal spray on solid substrates plays a role of fundamental relevance in the preliminary stages of coating design and process parameters optimization. The hole-drilling method is a versatile and widely used technique for the experimental determination of residual stress in the most superficial layers of a solid body. The consolidated procedure, however, can only be implemented for metallic bulk materials or for homogeneous, linear elastic, and isotropic materials. The main objective of the present investigation was to adapt the experimental method to the measurement of stress fields built up in ceramic coatings/metallic bonding layers structures manufactured by plasma spray deposition. A finite element calculation procedure was implemented to identify the calibration coefficients necessary to take into account the elastic modulus discontinuities that characterize the layered structure through its thickness. Experimental adjustments were then proposed to overcome problems related to the low thermal conductivity of the coatings. The number of calculation steps and experimental drilling steps were finally optimized.

Enhanced Lipid Extraction from Unbroken Microalgal Cells Using Enzymes

The marine microalga Nannochloropsis sp. was chosen as a model organism to investigate the feasibility of using cell wall-degrading enzymes to enhance the recovery of intracellular lipids. An enzyme cocktail containing galactomannanase, 1,4-β-cellobiosidase and β-glucosidase as main components was prepared from commercial enzyme preparations. The effects of pretreatment time (P), enzyme dosage (D), pH and temperature (T) on the amount of extracted lipids were investigated using response surface methodology. Under the best conditions (P = 90 min, D = 1.3 mg g–1, pH = 5, T = 36 °C) over 70 % of the lipids present in the microalga were recovered. SEM and TEM characterization of enzyme-treated microalgae showed extensive cell damage with significant disruption of the cell wall and release of algal material. Overall, the results obtained strongly support the use of commercial enzyme preparations to improve lipid recovery from microalgae and provide useful information on the influence of process conditions on the treatment efficiency.

Finite element analysis of residual stress in plasma-sprayed ceramic coatings

AbstractA numerical study was carried out by finite element analysis (FEA) for the calculation of absolute values and through-thickness variation of residual stress originating in thermal spray coa...

A plasma spray process for the manufacture of long-fiber reinforced Ti-6Al-4V composite monotapes

A fabrication method for titanium matrix composite monotapes reinforced by long SiC fibers is described. The plasma spray technique, carried out in an inert atmosphere, was used to deposit the metal matrix onto previously arranged continuous fibers.Major benefits are due to a controlled operating environment (the entire process is performed in a neutral gas atmosphere) and to the high solidification rate of the melted material. The formation of deleterious brittle reaction products between the fiber and matrix is therefore limited. Plasma spraying, normally used as a coating technique, was modified to produce a long composite monotape. This required a suitable arrangement of the fiber, placed onto a cylindrical substrate, and the identification of suitable operating conditions, as described in the present work. The results of characterization tests performed on the tape, with special reference to the quality of the fiber/matrix interface, are summarized. Results of preliminary diffusion bonding experiments carried out by means of a hot pressing system are also reported.