Unions have targeted the healthcare sector as a rare arena for expansion in recent years. But with Massachusetts hospitals growing increasingly cost conscious in a weakened economy, friction between management and organized labor is becoming more common. Members of the Massachusetts Nurses Association, who thought they had negotiated a more favorable retirement plan last October at four Caritas Christi Health Care hospitals, now find themselves at loggerheads with the hospitals' new owner, Boston's Steward Health Care System LLC, about putting in place terms of a plan the parties agreed to last fall. Across the state, Local 1199 of the Service Employees International Union has called on Baystate Health of Springfield to scrap plans to eliminate 354 jobs at the end of next week, arguing the hospital's projected budget shortfall will be offset by a Medicare windfall. Those clashes, following the nurses union's contentious contract bargaining last spring at Tufts Medical Center in Boston and St. Vincent Hospital in Worcester, underscore a new - more tense - environment as unions and hospitals stake their positions.