Chemical vapor deposition (CVD) is a well known method for depositing and forming a protective coating on substrates. Typically, the substrates are loaded into a reaction furnace (reactor), heated to a suitable reaction temperature, and exposed in the reactor to one or more elevated temperature gaseous reactants that react with the substrate surfaces to deposit a coating or layer thereon. The CVD deposited coating or layer can be reacted with the substrate by suitable heating in the reactor to form a protective diffusion coating thereon; e.g., a high temperature oxidation and corrosion resistant nickel or cobalt aluminide coating on nickel or cobalt base superalloy substrates as described in the Gauje U.S. Pat. No. 3,486,927.
As illustrated in that patent, the gaseous reactant may be formed in-situ inside the reactor for reaction with the substrates. Alternately, the gaseous reactant may be formed outside the reactor in a heated reactant generator and continuously introduced into the reactor via a carrier gas, such as a reducing or inert gas, so as to pass over the substrates. After passing over the substrates, the carrier gas and any excess, unreacted gaseous reactant are exhausted from the reactor to maintain a continuous reactant flow therethrough over the substrates. In one particular CVD coating apparatus, a plurality of substrates to be coated are positioned about a gas distribution conduit network within the reactor, and the gaseous reactant, such as a metal chloride or fluoride gas in a reducing or inert carrier gas, is formed external of the reactor in a reactant generator and is metered into the reactor via the conduit network for contact with exterior surfaces of the substrates. A separate gaseous reactant generator external of the reactor and separate distribution conduit network can be provided to supply and meter the same or different gaseous reactant into the reactor for contacting interior surfaces of the substrates if they are hollow. Both external and internal coatings can thereby be concurrently formed on hollow substrates, such as hollow gas turbine engine blades, using the CVD apparatus.
Typically, the gaseous reactant is supplied to the reactor in excess of the stoichiometric amount required for coating the substrates so as to insure that CVD coatings are formed on all of the substrates to be coated. As a result, the spent reaction gases that have passed over the substrates usually contain some excess, unreacted reactant that can condense at sufficiently cool locations in the reactor exhaust conduit and eventually plug or clog the exhaust conduit.
The problem of exhaust clogging or plugging is especially severe when aluminum subchlorides (e.g., AlCl.sub.2 and AlCl) and aluminum trifluoride and subfluorides thereof are used as the gaseous reactant and thus are present in the spent gas.
Plugging of the reactor exhaust conduit can adversely affect flow of the gaseous reactant through the reactor and even result in shut-down of the CVD apparatus in the event the exhaust conduit becomes completely plugged.
It is an object of the present invention to provide an improved CVD apparatus and method wherein the spent reaction gases are treated in a manner to condense any excess, unreacted gaseous reactant within the reactor prior to exhausting therefrom to avoid plugging or clogging of the reactor exhaust conduit.
It is another of the invention to provide an improved CVD apparatus and method system wherein the excess gaseous reactant condensed in the reactor is collected in a manner to avoid damage to CVD coated products in the reactor.